What is will in ethics?

The word "will" in ethics refers to human free will, which is simply our ability to make decisions.

It's important to praise or blame someone only when that person has an alternative. If humans lacked the freedom to make decisions, praising or blaming them would make no sense. That would be like praising or blaming a cloud for raining!

Being responsible for the consequences of an act (action, decision) to do or to refrain from doing something depends upon the freedom of the actor (agent).