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.