a common thing in the fandoms, that basically means a need to a couple, or in some cases more fictional characters to have a romantical
relationship. Probably comes from "
relationship" or in a reference to a ship(romantic feelings) drifting on sea(
relationship), and when it's sailing, the couple loves each other, but if it's sinking, their love is ending.
Ships have a names, that are shortcuts from characters that we want to be together. Mostly those names are from beginning of the character's names, and the one who's more dominating(seme in our opinion, is on beginning, and the one who's not (uke), in the ending, but it can be in different arrangement
Except for that, you'll need few more words to fully understand that, such as OTP, wich means that this is a couple we want to be together the most, and NOTP - the least.