Ju pli granda estas la rabato, des pli da homoj venas al la vendejo.

It is the Esperanto way to say the more ... the more ..., the bigger ... the more ..., and similar comparative pairings.

So:

• Ju pli granda estas la rabato, des pli da homoj venas al la vendejo.
• The bigger the discount, the more people come to the store.

A useful way to remember it:

• ju = introduces the changing condition
• des = introduces the matching result

You will often see this structure with pli or malpli:

• Ju pli vi studas, des pli vi lernas. = The more you study, the more you learn.
• Ju malpli oni dormas, des pli laca oni estas. = The less one sleeps, the more tired one is.

Because in Esperanto, ju normally goes directly with the comparative expression itself, here pli granda.

So the core idea is:

• ju pli granda = the bigger / the greater

Then the rest of the clause follows:

• Ju pli granda estas la rabato ...

Putting ju before la rabato would sound wrong because ju is not modifying the noun phrase la rabato. It is introducing the comparison.

Because pli modifies the adjective.

• granda = big
• pli granda = bigger

This is completely regular in Esperanto:

• bela → pli bela = more beautiful
• rapida → pli rapida = faster
• multekosta → pli multekosta = more expensive

So pli granda is simply the normal way to form bigger.

Because la rabato is the subject, and pli granda is a predicate adjective linked by esti.

In other words:

• la rabato estas granda = the discount is big
• la rabato estas pli granda = the discount is bigger

In the sentence, that becomes:

• Ju pli granda estas la rabato ...

The word order is a little rearranged for style, but grammatically it is still the same idea: the discount is bigger.

Because Esperanto usually uses da after words of quantity when talking about an amount of something.

• pli da homoj = more people
• multe da akvo = a lot of water
• iom da pano = some bread

Here, pli is functioning as a quantity word, so da is needed before the noun:

• des pli da homoj = the more people

For a learner, it is best to treat pli da + noun as the standard pattern for more + noun when you mean a larger quantity.

Because homoj refers to multiple people, so it takes -j.

• homo = person
• homoj = people

But granda describes la rabato, which is singular:

• la rabato = the discount

So the adjective stays singular too:

• la rabato estas granda
• la rabato estas pli granda

If the noun were plural, the adjective would also be plural:

• la rabatoj estas grandaj = the discounts are big

Because it refers to the discount in a general statement of the type the bigger the discount, the more people come.

Esperanto often uses la in this kind of structure, where English also uses the in a special comparative sense:

• the bigger the discount
• the more people

In Esperanto, only the first noun here actually appears with la:

• la rabato

But the whole pattern still corresponds to English the bigger ..., the more ...

So la is natural and expected here.

Because after da, Esperanto normally does not use la.

So:

• pli da homoj = more people
• not pli da la homoj

That is because da introduces an amount or quantity, and the noun after it is treated differently from a normal direct noun phrase.

Compare:

• multaj homoj = many people
• multe da homoj = much/many people, a large quantity of people

In this sentence, pli da homoj is the standard form.

Because the sentence expresses a general rule or usual tendency, not a single completed event.

Esperanto often uses the present tense for general truths:

• Akvo bolas je 100 gradoj. = Water boils at 100 degrees.
• Ju pli granda estas la rabato, des pli da homoj venas al la vendejo. = The bigger the discount, the more people come to the store.

So venas here means something like come / tend to come / will come in such cases, depending on the context.

Because al means to or toward.

• veni al la vendejo = come to the store

This focuses on the destination.

You might also see en la vendejon in some contexts, which would emphasize motion into the store. But al la vendejo is perfectly natural when the main idea is simply that people come to the store.

So:

• al = to/toward
• en ... -n = into

The ju ... des ... structure is fixed in principle, but Esperanto word order inside each clause is somewhat flexible.

So this sentence is natural:

• Ju pli granda estas la rabato, des pli da homoj venas al la vendejo.

You could also encounter variants with slightly different internal order, especially in speech or writing style, but the key thing is that:

• ju must introduce the first comparative idea
• des must introduce the second comparative idea

So the most important pattern to remember is:

• Ju pli ..., des pli ...
• Ju malpli ..., des malpli ...
• Ju pli ..., des malpli ...

No. It is very common with pli, but you can also use it with malpli, pli bone, pli rapide, and other comparative forms.

Examples:

• Ju pli rapide vi kuras, des pli laca vi fariĝas. = The faster you run, the more tired you become.
• Ju malpli oni elspezas, des pli oni ŝparas. = The less one spends, the more one saves.
• Ju pli bona estas la produkto, des pli alta estas la prezo. = The better the product is, the higher the price is.

So the pattern is broad and very useful, not limited to this one sentence.