A bitmap is a still image produced by a fixed matrix of colored pixels, like a mosaic. You can use bitmaps as textures for materials, as backgrounds to viewports, and as rendered environments.
You can use an animation or video file as a bitmap, in which case the material or background changes over time.
Bitmaps can be reloaded automatically after they have been changed and resaved by a graphic editing program. See the Reload Textures On Change toggle in Preferences Settings 
Files panel.