Rear brake not working properly

When I press it lightly it's not really working and when I press it all the way down, even though the wheel "locks", I can still spin it with my hand. I don't really know much about how the braking mechanism works unfortunately, I did clean it up around the area where you can tighten it up cause there was some debris but that didn't fix the problem.. I can still hear a bit of a metallic/grinding sound. Any sort of help will be appreciated..