Rim lock (also written as "rimlock") is an issue that can occur in magazine-fed guns that fire rimmed or semi-rimmed cartridges. Examples of such cartridges include .22 Long Rifle, .22 Magnum, .357 Magnum, 7.62x54r (rimmed) as well as .32 ACP and .38 Super (semi-rimmed).
The issue is that in a magazine-fed firearm, particularly when involving a detachable magazine or internal box magazine, the top most round in the magazine can sometimes slip backward behind the rim of the next round, causing the gun to seize up and be unable to feed the round from the magazine. Alternatively, the user can easily load a round in this way by accident. This is not really an issue in firearms with a tube magazine because the rims have no way of grabbing each other and locking up.
Preventing Rim Lock
In most firearms, rim lock is relatively easy to avoid. By taking care to make sure that each round's rim is sitting in front of the previous one, this should eliminate most if not all possibility of rim lock. However, it may be the case that some magazines are not designed to properly allow for this but give just enough room for cartridges to lock together and still cause issues. Overall, paying attention during loading will resolve almost all issues of rim lock.