GMT is local solar time in Greenwich, so it's an empirical estimate of "true" time that depends on the rotation of the earth. The problem is that the earth wobbles a little bit on its axis, so GMT varies a little tiny bit as a result. That's fine for humans setting their watches (or even marine chronometers), but most modern technology can't deal with that kind of variance. So our computers instead use UTC, which is defined in absolute terms and usually coincides with GMT, except when it doesn't. This helps to prevent the internet from crashing and planes and satellites from falling out of the sky.