Something that is barred prevents your entry. If you are trying to sneak into your friend's house, avoid the barred windows.
To be barred is to be blocked from entrance or not allowed to do something — as if there were imaginary bars in your path. The adjective barred comes from the noun bar, and it's easy to remember if you picture the classic barred cell where inmates are confined. Barred can also describe something that is marked with bars, like a barred owl.