Use "probably" to talk about something that you think is true, but you're not sure. "Probably" describes a probability anywhere from 50% to 99%. Use "probably" with an adjective:
You're probably exhausted.
You can also use "probably" with a longer phrase that acts like an adjective:
He's probably on his way.
And it also can be used with a verb:
They probably left to go get some food.