Rimlock is where the rounds get jammed in a magazine.
It happens with semi-rimmed cartridges like the .32 ACP or with fully-rimmed ones like the .357 Magnum (ala the Coonan pistol).
If the overall cartridge length is short enough, the round below the top one in the magazine can move forward. Then, the rim of the top round can slip behind the rim of the lower round, locking it in the magazine.
Can't happen with rimless or rebated rim cases like the 9mm, .40 S&W, .357 Sig and .45 ACP.