A flogger is a device or type of sex toy used to flog, or whip, a consenting partner, usually during a BDSM scene. Floggers typically consist of several tails attached to a rigid handle. The tails may be made from a number of materials, including horsehair, rubber, or leather.
To flog a person is to hit or strike a person’s bottom or back repeatedly with a flogger (a leather or rope instrument that looks similar to a cat-o-nine tails), while they hold onto or are tied to a wall or bed. In this case, the term "flogger" can also refer to the person doing the flogging. This is generally considered to be a BDSM activity, although it can be done by anyone possessing a flogger.
Flogging someone does not have to be something that causes the receiver pain in any way and it isn’t always an extreme sexual activity. In BDSM, the aim isn't to harm but to use pain to increase sensation, and therefore sexual pleasure.