Socialaj servoj gravas.

Because -o is the basic noun ending in Esperanto, and -j marks the plural.

So:

• servo = a service
• servoj = services

The ending -oj is just -o + j.

Because socialaj is an adjective describing servoj, and Esperanto adjectives must agree with the nouns they describe.

Here is the pattern:

• sociala servo = a social service
• socialaj servoj = social services

So:

• -a = adjective ending
• -j = plural

Since servoj is plural, socialaj also has to be plural.

gravas is a verb in the present tense.

• -as = present tense
• the verb is gravi

So gravas means is important / are important / matters, depending on context.

Esperanto verbs do not change form for singular vs. plural subjects, so:

• ĝi gravas
• ili gravas

both use gravas.

Because in this sentence gravas is a verb, not an adjective.

There are two common ways to express this idea in Esperanto:

• Socialaj servoj gravas.
• Socialaj servoj estas gravaj.

The first uses the verb gravi = to be important / to matter.
The second uses estas + the adjective gravaj = are important.

Both are correct. The version with gravas is shorter and very natural.

Because gravas already contains the whole verbal idea.

In English, we often say:

• are important

In Esperanto, that can be expressed either as:

• estas gravaj
  or more compactly as:
• gravas

So nothing is missing. gravas is a complete verb by itself.

Because Esperanto often leaves out la when speaking about things in general.

So this sentence is making a general statement about social services, not pointing to some specific ones already identified in the conversation.

Compare:

• Socialaj servoj gravas. = a general statement
• La socialaj servoj gravas. = those specific social services are important

Because there is no direct object here.

In this sentence:

• Socialaj servoj is the subject
• gravas is the verb

The accusative -n is mainly used for direct objects (and in some other special cases, like motion toward). Since socialaj servoj is the subject, not the object, it does not take -n.

Yes. Esperanto allows adjectives either before or after the noun.

So these are both grammatical:

• socialaj servoj
• servoj socialaj

But putting the adjective before the noun is usually more common and neutral, so socialaj servoj is the most natural choice here.

Yes, Esperanto word order is fairly flexible, but subject + verb is the most neutral order here.

Most natural:

• Socialaj servoj gravas.

A different order, such as Gravas socialaj servoj, is possible in some contexts, but it sounds more marked or stylistic. For a learner, the given order is the safest and most standard.

A helpful rough guide is:

so-tsi-A-lai SER-voi GRA-vas

A few useful points:

• stress falls on the second-to-last syllable
• c is pronounced like ts
• j is pronounced like English y
• oj sounds like oy
• aj sounds roughly like eye

So:

• socialaj → stress on A
• servoj → stress on SER
• gravas → stress on GRA

Yes. It is an international-looking word, so it is easy for English speakers to recognize.

That said, Esperanto still treats it like a normal Esperanto adjective, so it must follow Esperanto grammar:

• sociala = singular adjective
• socialaj = plural adjective
• socialan = singular accusative adjective
• socialajn = plural accusative adjective

Even familiar-looking words still take regular Esperanto endings.

Yes. It is a complete, natural Esperanto sentence.

It has everything it needs:

• a subject: Socialaj servoj
• a finite verb: gravas

So even though it is short, it is fully grammatical and idiomatic.