So… Every time you jump you have to add velocity or a force (of type impulse) upwards. But you have to stop doing this when you aren’t touching the ground. You can check if you are touching using BoxCast or Raycasts. Your current implementation of the jump is applying the jump(force/velocity) constantly when you press Space which as Nox mentioned below works like a jetpack, not like a jump. Try looking up “Ground Checking Unity” or something and you’ll get all info you need.
It seems like you are new to making games and by that standards, you did quite well. Keep learning and making cool games ❤️😉