Learn German: Hier ist ja Max' Spind, aber er ist völlig leer. - Max's locker is here, as you know, but it is completely empty.

In German, the genitive of proper names usually takes an -s with no apostrophe (like Annas Spind). However, if the name already ends in an s-sound (like -s, -ß, -x, or -z), adding another -s would be hard to pronounce. Instead, German just adds an apostrophe. Since Max ends in an -x (which sounds like /ks/), it becomes Max'.

What is the word ja doing in this sentence? Is it just "yes"?

Here, ja is a modal particle, not the word for "yes". It adds a flavor of "as you know," "as expected," or "obviously" to the sentence. The speaker is pointing out the locker, implying that both they and the listener already knew this was Max's locker.

Why does the second half of the sentence use er to mean "it"?

In German, pronouns must match the grammatical gender of the noun they replace. The word for locker is masculine (der Spind). Therefore, to say "it is completely empty," you must use the masculine pronoun er (er ist völlig leer).

Could I use ganz instead of völlig for "completely"?

Yes, you could absolutely say aber er ist ganz leer. Both völlig and ganz can mean "completely" or "entirely" in this context. Völlig just sounds slightly more emphatic, like "absolutely empty" or "totally empty".