I dont understand how isGrounded works. In a 3d game.
IsGrounded checks if the player is on the ground, usually, when you press the jump button (say space bar in this case)
It will say either (!IsGrounded) // not grounded or IsGrounded = false.
This will prevent “Double jumping” unless you want this to be a feature.
But if you are talking about the Unity character controller I think this is what happens.
I prefer to code my own character controller, so i can change things easier 