People use "you guys" when talking to a group of people that may include men and women. For example, a waiter might say this to a large family eating at a restaurant:
Have you guys been here before?
English has a problem because "you" is used for both one person and a group of people. It can be confusing. To fix this, people in different regions use different words to talk to a group. In Southern areas of the U.S., some people say "y'all":
Have y'all covered exponents yet?
People in other areas of the country think this sounds really dumb, though.
"You guys" is still considered casual English, but it's very common for younger people and people from the West Coast of the U.S. to use this phrase.