A continuous light built into many professional flashguns so that you can see the effect of the light ahead of taking the picture. Without a modelling lamp, you won’t really be able to gauge the effect of the lighting without taking a shot and looking at the result.
Traditionally, modelling lamps were much lower-powered than the flash unit itself and not designed to provide illumination for taking pictures on their own. However, the increased use of video and its need for continuous lighting has mean that flash makers are now incorporating LED lamps which can be used for this kind of continuous lighting application.