Follow is nothing but forward motion on the cue ball after contact with the object ball. Imagine a straight forward blue from near the pink spot with a loose red in the baulk area. You will have to hit the cue ball with low to medium speed and high to medium amount of follow to get there and be able to play the red.
How do you achieve good follow? Simply put, keep your hands loose, increase the distance between your hand and a cue ball by a little bit, and hit it higher than normal to make the cue ball travel forward. When executing a follow shot, don't try to hit the cue ball harder than normal; instead, concentrate more on the amount of follow you have put. This rule can be broken if you want to break a cluster of balls, which is an advanced concept, and if you are a beginner level player, you can just skip it for now. I will try and put an article on the high force cluster breaking shot after potting the black with follow in a few days. You can check it if you think that you know all the basic shots, and your game is good enough to move on.
There is another shot, which is similar to a follow shot, but that is with just a little bit of follow. It is known as stun run through. In this shot, the more the speed and the distance, the less amount of follow you should put. This shot is extremely important in certain situations. For example, you have a 3 quarter hit on the blue in the center, on the proper side to break the cluster of reds without hitting any baulk cushion. There is a loose red near the black pocket ahead you, which can be reached by playing a stun run through on the blue. You can also go in the cluster, but play the stun run through if breaking the cluster feels too risky. This shot is somewhere in between stun and follow, closer to stun, that's why the name stun run through.
A little analysis of the follow shot.
Try it out on the table. It is very easy once you get the hang of it. In my opinion it is the easiest of the three; stun, screw back and follow.