Used in conditional expressions, often as part of もしかして or もしかすると, meaning 'perhaps' or 'if by any chance'.
もしかして、彼はもう来ているかもしれない。
Perhaps he has already arrived.
もしかすると、雨が降るかもしれないから傘を持って行こう。
In case it rains, let's take an umbrella.
もしか is another bound adverb that, like ひょっと, mainly appears combined with すると/したら/して to hedge a guess, though もしか sounds slightly more common in casual speech.
Derived from もし (if) with the addition of か (question marker), forming a tentative conditional expression.