over the course of time, files on your hard drives become 'fragmented' - bits of them get scattered around all over the place.
if you're old enough to remember vinyl records, you will also remember that the audio tracks were arranged more or less in concentric rings around the disc - the first inch from the outside was track one, the second inch was track two etc. that is an example of a perfectly defragmented disc.
now imagine if the first five seconds of track one were at the edge of the record, but the next five were half way into the disc, the proceeding ten seconds were right in the middle and the rest was all over the place. this is severe defragmentation.
in order to play the record, the record player's head has to jump about all over the place, taking time and wearing itself out.
hard drives are similar in this respect - if you defragment your computer, all the files end up being arranged nice and neatly on your drives, and so theoretically load up quicker.
in practise, you probably won't notice that much difference, but why not set the defrag running before you go out or something, and let it do its thing.