An easy translation for this is "to attack". Even when you are not physically striking your opponent but mentally you are putting pressure on your opponent it is also called "seme".
It is hard to grasp the concept, so I will introduce some teachings in kendo.
In order to apply a good seme in kendo, you must know how you actually conquer your opponent.
(Three Things to Kill)
You must get your opponent's sword away from his/her center. "To kill" means here is "to disable" your opponent's sword. As long as your opponent keeps his/her sword in the center you cannot strike them.
If you strike them while their sword is still in the center, you will be one good kebab. So you must use all sorts of techniques to get your opponent's sword away from their center.
Some useful techniques to kill your opponent's sword.
Make sure that you will not give your opponent any opportunities to attack you.
For example, you can use the techniques introduced above (osae, harai or maki) right at the moment when your opponent moves to strike you. It messes up their timing to execute their strike.
You can always initiate the move by anticipating your opponent's move. In this way, your opponent does not have any opportunities
to execute their technique on you. As a result, they get frustrated and give you an opportunity to strike them.
You opponent must be very enthusiastic about beating you. You discourage your opponent. How do you discourage your opponent? You can make yourself look big and strong.
If your opponent chickens out because you look strong, then you discourage your opponent from fighting with you. If you can scare your opponent with your kiai, that also works.
If you are 3rd dan or above, try to put pressure (ki) on your opponent. While fighting to take the center, try to create shikai in your opponent's mind. You can do so by killing your opponent's sword and techniques.
San Sappo is an important element of seme, so first learn to kill your opponent sword and techniques. There are easy because you can actually see the sword and opponent moving.
To kill opponent's ki with your ki is the hardest thing to do, so if you cannot do it or get what it means, do not worry. You will get there eventually.
If you hear someone yelling "seme! seme!" at you in shiai, they are suggesting that you should go forwards, create some opportunities to strike and attack.