Imputation in law means that a person can be held responsible for something that someone else did because of their relationship or connection to that person. Imagine you and your friend are playing catch with a baseball. Your friend throws the ball and accidentally breaks a window. If your parents hear about it, they might say "You are responsible for that broken window because you were playing with your friend and it was your combined activity that caused the accident." This is imputation - you are held responsible for something your friend did because of your relationship and joint activity.
In legal proceedings, imputation can come up when a person or group is held responsible for the actions of another person or group. For example, a company might be held responsible for the illegal actions of its employees if it is shown that the company encouraged or allowed the behavior. Or a parent might be held responsible for damages caused by their child if it can be shown that the parent should have known what the child was doing and failed to prevent it.
Overall, imputation is a way of holding people responsible not just for their own actions, but for the actions of those they are connected to in some way.