Like a lot of kendo terms there is at least two meanings to shomen: a simple one and a more subtle one.
The simple meaning of the term shomen refers to cutting the centre of your opponent's head. It is the first cut everyone learns. To be precise this is called shomen-uchi 正面打.
The more subtle meaning is to face something head-on. This is why it is so important in kendo, because kendo is not about learning how to defeat another person but to develop your self. So in kendo you need to practice facing things head-on, not shying away or making excuses.
The basic technique in kendo is shomen uchi. Using a straight backswing and a straight cut, you aim to strike your opponent's men (helmet) right above the tate-gane, or vertical bar of the mengane (helmet's steel grill).
A UMKC student asked me recently, "when should you use sa-yu-men?" (left or right diagonal men cuts). This made me think and I realised something about shomen.
It is often said that all the techniques in kendo comes from shomen, and that if you can execute shomen confidently you will be able to master all the other techniques. Why is that? Well, because if you see all the different techniques in kendo as separate you will learn them all separately, and they will remain separate for you. As a result you will probably specialise in just one or two techniques that you can do really well, because trying to learn all the others just takes time away from perfecting your 'preferred waza'.
This is the wrong approach.
You should see shomen as no different to all other techniques. Sa-yu-men, kote, do and, later on, tsuki, are all just variations of shomen. If you perfect shomen you can perform all the others as variations of shomen. You adapt shomen on the spur of the moment to become kote, for example. Seen in this way, when you practice your shomen, you are also practicing all the other techniques, and vice versa: when you practice the other techniques, you are also practicing shomen.
How and why does this work?
As I mentioned earlier, the term shomen has the feeling of facing things straight on. In kendo if you face your opponent straight on without flinching or backing down, you will eventually break through. If you face them with a single-pointed focus, and a completely empty mind, then if the opportunity arises for kote instead of shomen, you will perform kote easily. If, on the other hand you have practiced kote as a separate technique to be used in certain situations, then your mind will have to sort through all the waza you know and choose the right one before you can act. In kendo, this is too slow.