maybe is an adverb, meaning perhaps or possibly. may be is a verbal construction, formed of the modal verb may and the verb be, which can be used in sentences like “he may be in the office today”. Note that, if you were to rewrite this sentence with maybe, you would say “maybe he is in the office today”. Because maybe is an adverb, you need to add a verb to the sentence (is, a form of be, in that case).

"Maybe" is an adverb. "May be" is two verbs, "may" and "be". It is the same as "could be" or "might be", except it sounds more formal. E.g. I may be late tomorrow. Maybe I'll be late tomorrow.