“They have a screened-in porch in back. ”
You visited your boss's house, which was nice. Now you're describing the house to your boyfriend. You tell him about an outdoor area that's protected from bugs.
They have a screened-in porch in back.
When something is "screened in", it's surrounded by a screen to keep out insects. A "screen" is like a fine net made out of thin metal wires.
Some homes have a "screened-in porch", which is an outdoor seating area that's surrounded by screens.
"In back" can describe the area behind someone's home.
The front yard has a wide lawn and there's a garden in back.
When you're talking about a store, "in back" means a room that's further away from the front door. Often it means an area of the store where only employees are allowed to go.