A MIDI controller is a type of input device used to send MIDI data to a computer or another type of device. In simple terms, it triggers sounds from an external source. A MIDI controller essentially allows you to control musical hardware or software in a similar way to using a keyboard or mouse to control your PC.
There are various types of MIDI controllers, from keyboard controllers to drum pads, and they work with all kinds of sources, including synthesisers, mixing desks and virtual instruments within a DAW.