In short, “OTP” stands for “One True Pairing.” It's used in fandoms to describe any given participant's favorite couple — or couples, because, perhaps contrary to the term's very definition, you don't have to limit yourself to just one OTP (and no, they don't have to be canonical). Usually, though, the characters or people who make up any OTP will not occur in more than one pairing for any single fandom member.
Less commonly used is the term TTP, or Two True Pairings; if you can't make up your mind about an OTP, then TTP might be substituted instead. And if you're into romantic arrangements that involve more than two people? Try OTT (“One True Threesome”) on for size. Other variations include BroTP — a portmanteau between “bro” and “OTP” that refers to a non-romantic OTP — and parodic versions. The example TV Tropes gives of this last one involves shipping a character and their favorite food.