* "Moment": As in "Just give me a mo to think."
* "Month": As in "It's been a long mo since we've seen each other."
* "More": Informal, particularly in spoken language.
It could also be:
* A person's name: There are people with the first name "Mo."
* A slang term: "Mo" is slang for "money" in some contexts.
To understand the meaning of "mo," you need more context. What was the sentence or situation where you encountered it?