probably is an adverb that is used to indicate a higher degree of likelihood of an event happening or being true.
probably is usually placed in the middle or at the end of a sentence. Probably is used to show an expectation for something to happen or to indicate the degree of certainty of a statement is high.
1. It will probably rain tomorrow.
2. He probably knows the answer to the question.
3. She will probably be home by 7-00p.m.
In informal situations, you can omit probably and just use the main verb. For example, instead of saying 'They will probably do it,' you can say 'They will do it.' To emphasize uncertainty, you can replace probably with possibly or maybe.