J’ajoute un peu de levure pour que la pâte soit plus légère.

This is called elision. In French, je becomes j’ before a word that starts with a vowel sound, like ajoute.

So:

• je ajoute ✘
• j’ajoute ✔

The same thing happens in many common phrases:

• j’aime
• j’habite
• j’écoute

J’ajoute is the present tense of ajouter.

In French, the present tense can often mean both:

• I add
• I am adding

The exact meaning depends on context.

So J’ajoute un peu de levure... could mean:

• I add a little yeast...
• I’m adding a little yeast...

French does not usually need a separate form like English am adding.

After expressions of quantity such as un peu de, French normally uses de by itself.

So you get:

• un peu de levure = a little yeast
• beaucoup de farine = a lot of flour
• assez de sucre = enough sugar

If you were not using a quantity expression, you might say:

• J’ajoute de la levure = I’m adding some yeast

So the pattern is:

• de la levure = some yeast
• un peu de levure = a little yeast

Pour que is used when it is followed by a conjugated verb with its own subject.

Here, the second part is:

• la pâte soit plus légère

That clause has its own subject: la pâte.

So:

• pour que la pâte soit plus légère = so that the dough is / may be lighter

By contrast, pour is used before an infinitive:

• J’ajoute de la levure pour alléger la pâte = I add yeast to make the dough lighter

Both are possible, but the grammar is different:

• pour + infinitive
• pour que + subject + conjugated verb

Because pour que normally requires the subjunctive.

So after pour que, you do not use the ordinary present tense est. You use the subjunctive form of être, which is soit.

Compare:

• la pâte est légère = the dough is light
• pour que la pâte soit légère = so that the dough may be light / becomes light

This is one of the most important subjunctive patterns in French:

• pour que + subjunctive

Soit is the third-person singular present subjunctive of être.

The present subjunctive of être is:

• que je sois
• que tu sois
• qu’il / elle soit
• que nous soyons
• que vous soyez
• qu’ils / elles soient

In this sentence, the subject is la pâte, which is singular, so French uses soit.

Because légère agrees with la pâte, which is feminine singular.

In French, adjectives usually agree in gender and number with the noun they describe.

So:

• un pain léger = a light loaf/bread
• une pâte légère = a light dough

Here:

• la pâte = feminine singular
• therefore légère = feminine singular

French usually forms the comparative with plus + adjective.

So:

• léger = light
• plus léger / plus légère = lighter, more light

This is the normal way to say -er adjectives from English:

• plus grand = bigger
• plus rapide = faster
• plus simple = simpler

So plus légère is just the regular French way to say lighter.

It can absolutely just mean lighter in general.

French often uses plus + adjective even when the second item in the comparison is only implied. In this sentence, the idea is:

• lighter than it would otherwise be
• lighter than before
• lighter in texture

So plus légère does not need an explicit comparison such as than before.

Here plus means more, not no more / no longer.

In plus légère, the usual pronunciation is often just ply, because the next word begins with a consonant:

• plus légère ≈ ply le-zhair

In careful speech, some speakers may pronounce the s, but ply is very common here.

A useful thing to remember:

• before a vowel sound, plus often links as plyz
• before a consonant, it is often just ply

Here que is part of the fixed expression pour que, which means so that or in order that.

It is not translating a separate English word by itself in a very direct way. Instead, you should learn pour que as one unit.

Examples:

• Je parle lentement pour que tu comprennes.
• Il ferme la fenêtre pour que le bébé dorme.

So in your sentence, pour que introduces the purpose:

• the reason for adding the yeast is that the dough should become lighter.