A daughterboard (also sometimes called a "daughtercard") is an add-on card which provides additional functionality, for example MPEG decoding or video capture to a video card, or wavetable sound to a sound card. While they are going out of fashion in recent years due to the generally low cost of computer hardware - why upgrade when you can replace for about the same price - they have not entirely disappeared.
Daughterboards usually attach to their host via a header connector, generally a double row of pins which stick out from the host device. You can see a male header connector on the back of the average hard drive, and a female header on the average hard drive cable.
For contrast, compare the term motherboard.