A typo, short for typographical error, is a misspelling produced by careless typing. Leaving more than a few of these in your fic is a sure sign that you didn't engage the services of a competent beta-reader.
Some typos result in gibberish, which will usually be caught by a spell-check program. Others, however, result in real words that mean something the author did not intend. This kind of typo can wreak all sorts of havoc in a Word World, ranging from unintentional comedy to unintentional horror. Typos of canon names generate different minis depending on which continuum the name is from.