Learn French: Ses amis ne comprennent pas la question. - Her friends do not understand the question.

In French, the possessive word matches the thing being owned, not the owner! Because amis (friends) is plural, you must use the plural possessive ses. It doesn't matter if the owner is a "he" or a "she".

If ses just tells us that the friends are plural, how do we know the translation is "her friends" and not "his friends"?

Looking at just this one sentence, you actually don't! Out of context, ses amis could perfectly mean "his friends" too. In a real conversation, you would already know who we are talking about from the previous sentences.

How does the negative "do not" work here with ne and pas?

To make a sentence negative in French, you put ne before the conjugated verb and pas after it, like a sandwich. Here, they wrap around the verb comprennent to make ne comprennent pas (do not understand).

What verb is comprennent and why does it have such a long ending?

It comes from the verb comprendre (to understand). When you are talking about a plural "they" (like "her friends"), verbs take the -ent ending. Just remember that in French, this -ent verb ending is completely silent!