Photo / Video
Full-frame
A camera sensor about the size of one 35mm film frame, larger than APS-C for better low-light shots.
A camera sensor roughly the size of a single 35mm film frame (about 36×24mm). Because the sensor is physically larger than APS-C or Micro Four Thirds, it captures more light for a given exposure, which generally translates to cleaner images in low light and the ability to render a shallower, more out-of-focus background at the same field of view. Those advantages typically come with higher body and lens prices and more weight to carry.
Concrete example
at the same aperture and framing, a full-frame shot will usually show more background blur than an APS-C one.
What it means when you're buying
choose full-frame if low-light work or maximum background separation matters most, and APS-C or smaller if budget, size and reach are higher priorities.