A speedloader (typically made for direct reloading of revolvers) is a device used for reloading a firearm more quickly. For a revolver it is often a case that holds a full capacity of bullets that is pressed against the cylinder to reload it.
In Video Games
Speedloaders are typically objects that the player doesn't have to obtain as they are automatically on their character at all times. For characters that fire revolvers and reload them quickly a speedloader is often realistically modeled and shown. Sometimes the characters reload the revolver as if they have a speedloader but it isn't modeled and shown.
For other characters that load the revolver manually they don't have a speedloader. Characters without speedloaders/speedloader looking reloads are at a disadvantage because the longer they take to reload the longer they are a vulnerable target that currently can't fight back for the enemy.