Green Front Inn is a South Side staple, I'm pretty sure it's existed about as long as I've been alive and I'm in my 30's.
It's a very odd combination but Green Front is a dive bar mixed with an "old people" restaurant. The latter comment is the biggest hint about the overall food quality: most things aren't very flavorful and are usually unsalted, but everything is also incredibly cheap plus they have daily specials.
Green Front shines with their sandwiches. They're known for their fried fish, but they also make surprisingly good Reubens and steak hoagies (my Green Front meal of choice).
The servers are friendly if not a bit confused, ordering can be an inconsistent experience and I usually have to make extra sure that they have my order right before they take it to the kitchen. The staff also do not always talk to each other very nicely but in the same vein you can tell that they've worked together for years.
If you live in South Side, Green Front is worth a try to say that you got to experience the Green Front, but definitely don't go in expecting to be wowed.