To head off means to prevent something from happening by acting early, or to intercept someone by getting ahead of them.

When you head off a problem, you stop it before it gets worse. If your teacher notices that two students are starting to argue, she might head off a bigger conflict by separating them and helping them talk through their disagreement calmly. If you see dark clouds forming while playing outside, you might head off getting soaked by going inside before the rain starts. Smart planning often means heading off problems before they arrive.

The phrase also means physically intercepting someone or something by moving to get in front of them. If your dog escapes the backyard and runs toward the street, you might try to head her off by running ahead to block her path. Cowboys on horseback would head off cattle that strayed from the herd by riding ahead and turning them back.

Notice how both meanings involve getting ahead of something: either ahead in time (preventing a future problem) or ahead in space (blocking a path). When someone says “Let's head off this issue now,” they mean dealing with it immediately, before it grows into something harder to fix.