A centre speaker is, as its name suggests, the centre channel for a home theatre setup and placed in the front of a room, in the centre of the front left and right speakers.
The centre speaker is responsible for most of not all of the dialogue in a movie; it simply reproduces the audio intended to come from immediately in front of the viewer.
Generally, centre speakers comprise two mid/bass drivers and a tweeter, such as the Wharfedale Diamond 10.CS or Crystal CR30.C. Some more high-end models have a tweeter, dedicated mid-range and two bass drivers, like the Wharfedale Opus² Centre and in some extreme cases a centre speaker can have a tweeter, dedicated mid-range and four bass drivers, like the
Wharfedale Jade C2.