A shooter is a kind of video game. The aim of the game is to beat enemies by shooting (or otherwise killing) them. The enemies shoot back. The aim of the game is to stay alive as long as possible.
Many of the oldest computer games were shooters; the first video game ever made was a shooter called Computer Space. And one of the first games that many people played was a shooter called Space Invaders.
There are lots of different kinds of shooter. Now many people like first-person shooters. But there are other kinds too. In Japan many people play shooters where the enemies fire lots of bullets. The bullets make beautiful patterns on the screen. This kind of shooter is called a barrage shooter or a curtain fire shooter. It is also called by the Japanese name, danmaku.