What's voulez mean? and why it has letter "z" at

what’s voulez mean? and why it has letter “z” at the end?

I think you should have at least a quick look at the grammar in the beginning of your learning. Stephen Kaufmann advises to get a short, simple introduction to the language first, without dwelling on the explanations much and then go on to reading texts. He later goes back to that intro. I think he likes the “Colloquial” series (I myself am more into Assimil, but that’s longer).

Short explanation. “Voulez” means “want” as in “you (plural) want”. In English you must add an “s” to verbs at the third person singular, He “wants”, In French you add different endings for each person. “Z” is typical for second person plural (y’all), also used to refer to a single person in a polite way.
Verb vouloir (to want) in the present tense:
I want = Je veux
You want (singular, familiar) = tu veux
He/She wants = Il/Elle veut
We want = Nous voulons
You want (plural / polite) = Vous voulez
They want = Ils/Elles veulent