Just as you'd think, a large bell which is placed around the neck of a cow. The purpose of the bell is to allow the cow to be located if she tends to wander off.

The distinctive note of the cow bell is also used in some country music songs.

A nodeshell rescue.

It was popular enough at the time to be included in the TR-808. The synthesized 808 cowbell sounds nothing like a cowbell (it has too long of a decay and sounds too springy), but it has a distinctive sound that has become quite popular on its own. Most drum machines and synthesizers today include a cowbell, sometimes even multiple variants such as "low cowbell" and "high cowbell." My personal favorite, besides the 808 cowbell, is the Alesis D4 "heifer" cowbell. It's a high cowbell that has a similar tone as the 808, but actually sounds more realistic.