IMHO is internet shorthand for 'in my humble opinion'. You drop it in before you say what you think — in email, a forum, a comment, a chat — so the other person knows this is a view, not a fact. The 'humble' is a politeness costume, and quite often a joke. People who feel very sure of themselves still write IMHO. It belongs to informal writing on a screen. You would not put it in a school essay, a job letter, or a talk in class. Next to it sits IMO ('in my opinion'), which is the same idea without the fake-modest extra word. Most people say the letters out loud, 'I-M-H-O'; a few read it as a little word, 'im-ho'.