Seme is one of the most complex issues in Kendo and as such I would not presume to try to explain it in detail. For a clear understanding of seme, I would refer anyone to Dr Sotaro Honda’s article on tactics, published on the BKA official web-site. http://www.kendo.org.uk/pmwiki.php/Main/Tacticsinkendo
I am aware however, that seme is understood on a variety of levels and means different things according to your position in your kendo career. At it’s simplest “semeru” means to attack or assault and if you take the meaning at face value, it is a pretty good definition. You attack your opponent with your mind and spirit, forcing them either to retreat or attack before they are ready, thereby giving you the opportunity to make an effective strike. This seme is of course, where the point is won or lost and is far more important than the following waza. I remember watching a 9th dan mohan geiko, where the point was acknowledged without being struck, as it was clear to both parties, that there was no recovery after a successful seme.
Many beginners make the mistake of pushing the tip of the shinai forward with only their hands. This is unlikely to make any but the least experienced kendoka react, whereas a strong, (but small) movement with the right foot, followed immediately by the left, allows you to bring the whole strength of correct chudan to threaten your opponent. You should feel as if you are making this attack from your centre and ensure that your posture is correct and relaxed, being careful not to drop the point of your shinai as you step into your opponents distance. Do not step in too deeply -your shinai should ideally not go beyond 15cm of his kisaki.
Another common fault is to think of the seme as attached to the following technique, so that seme men is done in a continuous movement. At the point of starting your seme, you are not sure how your opponent might react. He or she may start an attack giving the opportunity for degote or debana men, or step back allowing a tobikomi men attack or try to strike your men quickly, giving the opportunity for kaeshi dou. The variations are numerous, so you should not have a set game plan. Instead push in and hold your concentration, in an attitude that is referred to as “tame” (holding back or saving) and then release the appropriate waza, when you see the chance.
Many people refer to seme as pressure, which is true, in a way. However this does not mean that your movement should be stiff or heavy. One useful tip is to think about applying seme from your navel upwards to your opponents nodo. This allows you to make the following attack in a positive forward motion.