Here's why:
* It modifies a verb, adjective, or another adverb. For example:
* "She will probably go." (modifies the verb "go")
* "He is probably tired." (modifies the adjective "tired")
* "They are walking probably too fast." (modifies the adverb "fast")
* It expresses likelihood or certainty. It tells us how likely something is to happen.
Let me know if you have any other grammar questions!