It is a casual, relaxed place, located in Coyoacán (trendy/hippy area), where you can just sit, have a beer, mezcal or tequila with your friends, and let the time pass with no hurry... if you are lucky, you will even be able to hear some mariachis in the plaza
It is a place where only locals hang out, if you wanna have a taste of mexican drinks, food and environment, this is the best place... My recommendation would be to go at night... you can spend hours there just chilling and chatting with friends
Try mezcal!! You cannot leave Mexico without trying it!! They also have some "cervezas artesanales" (hand craft beers), go for one!