A meat grinder is a style of melee combat. The term can mean either a generic battle that creates high losses in a closely packed area, or a particular melee format. The latter, which appears to a regional usage, is addressed here.
Of course, it also refers to a device designed to grind meat, such as is used in the making of sausage.
A meat grinder is a melee format consisting of two teams, generally placed on either end of a bridge of reasonable width. The first team is composed of experienced fighters, while the second contains newer fighters. The experienced team gets a single "life" (no resurrections), and the newer fighter team receives unlimited resurrections. The goal of the newer fighter team is to eliminate the experienced fighters. The experienced fighters attempt to hold off this outcome for as long as possible.
Variation can be easily introduced into a meat grinder. After one or two rounds, it is usually clear what changes can be made to even the sides. Often it is helpful to place one experienced fighter on the newer fighter's team. This fighter can help with cohesion and encouragement. The experienced team can also be given a limited number of resurrections if necessary.
The meat grinder offers multiple benefits as a training tool. The newer fighters learn basic melee fundamentals (sticking together, approach speed and distance, throwing shots in close quarters, etc), and also get the chance to learn in a progressive manner and try new things, as they have unlimited resurrections. Hesitation at fighting experienced fighters can also be cured. The experienced team learns to act conservatively, since the loss of even one member can be highly detrimental. Both teams gain experience dealing with lenghthy (relatively) battles requiring high stamina.
This format can seem harsh to newer fighters. It isn't a gentle method, and confidence can wane if the newer fighter team cannot make a dent in the experienced fighters. Conversely, the experienced fighters may find it tedious.