I tested the outlets with an AW Perry 3-prone tester in my study, all 3 lights lighted up. What does it mean ?

The wiring in my study is acting weird. When the prior owner of our house converted the garage to a room, he hired a handy man to make the changes. This guy apparently messed up the wiring. The problem was not fixed when we bought the house. Last week, my computer was fried, even with an UPS. So I tested the outlets for the wiring problem, all 3 lights lighted up on the AW Perry 3-prone tester, which the tester does not have an explanation. I don't have a clue as to what the problems are and how to fix them. Any suggestion is very much appreciated.