Sperm motility is vital for fertility. In order to achieve their goal of fertilising an egg, sperm have to swim strongly enough to cover what is, given their tiny size, quite a marathon. Nature has equipped them with a fructose rich energy source in the seminal fluid to help them on their way, but if the sperm are not fit enough for the job they will not succeed.
A good proportion of healthy, motile sperm is therefore essential. In humans the proportion should be at least 50%, and if it falls too far below this, either poor fertility or total infertility will result.
‘Motile’ simply implies that the sperm can move, but in fact that isn’t enough. To reach the egg they need to be able to move in a strong forward motion. Many sperm can wriggle their tails but not actually move forwards. Others go in zig-zags. While some don’t move at all. Only the forward moving sperm have a high likelihood of achieving fertilisation.
Category 1 are obviously the sperm that give a man high fertility, and a healthy male should produce large numbers of these.