"y'all" is short for "you all" and this is a casual way to distinguish between you (one person) and you (many people). It is also used in some of the southern parts and more rural parts of the US very frequently in common speech. Example: You walk over to your friend Anna who is sitting next to some of her friends. If you say to Anna "Hey, some of us are going for tacos, you want to come?" this means Anna is definitely invited, but it's unclear if her friends are. If you say to Anna "Hey, some of us are going for tacos, y'all want to come?" this means Anna and her friends are all invited. There was a popular tv show set in the country (the rural areas, not the urban areas) where the host said good-bye by saying "Y'all come back now, ya hear?" which is slang for "You (plural) come back later (to watch the next show, with imperative emphasis)" If you are learning English, this is good to know and understand, but not very useful to say and write (since it is slang). Hope this helps!

y'all means "all of you who think"