An impetus is the force behind something, whether it's a boulder rolling down a hill or a person making a decision.
Very little would get done if there were no such thing as an impetus: an impetus is some kind of force that gets something or somebody moving. If you push a car that's out of gas, you're the impetus that's getting it moving. An impetus doesn't have to be physical. Advertisers hope their commercials will be an impetus to buy the product.