The phrase "to let up" means to stop, relax or ease off. It can be used in various contexts such as physically, emotionally or metaphorically. For example, if someone is working out and they are doing an exercise that requires them to push against something with force, the trainer might say "let up" meaning to release the pressure and take a break from pushing so hard. In another context, it could mean to stop being angry or upset about something, to forgive and forget. It can also be used metaphorically in situations where someone is trying very hard to achieve something but they need to relax their efforts for a while before continuing again.