Digital trim can mean two different things, though one meaning is a lot more common.

What it normally means is that instead of having a mechanical lever that moves to "trim" or adjust the center point of the sticks, there is a little button or knob instead. You tap it one way or the other to trim the plane, but the knob doesn't actually move.

The big advantage to digital trims is that since the lever or knob never moves on the TX, you trim the plane in flight, and forget about it. The trim setting is stored in the computer's memory, and it won't change when you turn the radio off and turn it back on.

On analog trims, you have a mechanical lever that moves to set the trim setting. The problem with this is that if you turn off the TX, and then bump the trim levers, the plane will be out of trim the next time you take off. You have to do something manually to re-center the trim levers, so that the plane is trimed for straight and level flight with the trim levers in the center positions (which is the ideal situation).

Another drawback to analog trims is that if you have multiple models on the radio, if one plane requires 3 clicks of downtrim to fly level, and the other requires 5 of left aileron, then you have to remember this, and reset the levers when you switch from model to model. On a digital trim system, the trim settings should be saved with each model, so the plane is always trimed out, and it's harder to take off with a model badly out of trim.

Some people prefer the feel of mechanical trims (me among them), and there are some radios that have the feel of mechanical trims, but have a way to "save" and re-center the trim levers to avoid the problems with knocking the levers out of possition.