When you are charged with a crime you have three options for a plea: guilty, not guilty, and no contest.
A guilty plea says you admit to the crime.
A not guilty plea says that you do not admit guilt, and you want your right to a trial. Not guilty pleas go to trial unless changed, as prosecutors only make plea deals with people who plead guilty or no contest.
A no contest plea says you agree to accept conviction but you don’t admit guilt. You’re basically saying: based on the facts you have gathered I see how you could come to this conclusion. I didn’t do it, but I admit that you could convict me if you went to trial.
In some cases a no contest plea is treated exactly like a guilty plea. It is also usually entered as part of a plea deal.