To be given a caution, the police officer must have evidence that you have committed a crime, which has enough evidence that would lead you to being convicted if it went to court. You can only get a caution if you admit to doing it and have not committed any other crime in the past. If you do not admit to it, you can be arrested and charged.
You can also be given a conditional caution which is similar to a caution, but you have to commit to things the police ask you to do, like getting help for alcohol or drug addiction.
A caution isn’t a criminal conviction, so once it is complete, it is considered finished with , but if you commit further crimes where you end up in court, it can be used as evidence to show that you are repeatedly offending.