If something probably will happen, it's likely. If it probably won't happen, don't get your hopes up.
This is a word for things that are likely to happen or just believable. If your dinner is gone and your dog looks guilty, the dog probably jumped on the table to eat it. If someone moved all the furniture around while you were sleeping, you probably would have noticed. Given all the billions of stars in the sky, there's probably life out there, but who knows if we'll ever meet it. You probably like some foods better than others.