If water starts backing up into your drains or toilet when you are flushing or running water, then there is likely a blockage somewhere in the sewer line. The easiest way to tell if the blockage is at the clean-out is to pour a bucket of water into it. If it backs up into your sewer drain or toilet, then you have a problem.
The line may be clogged because there is too much water pressure coming into the house or perhaps someone mistakenly dropped something down into the drain that shouldn't have been there. Dogs are also known to dig holes by the clean-out and cause blockages.
Unclogging your toilet can be done at home with the right tools, but you should really get a professional to look at your sewer cleanout if it is not working properly.