Denying creeps is just the act of forcing an attack on your own creeps that have fallen below half health. It is a very important part of the game but not something you should worry too much about when you are starting out to learn the game. All you have to do is press a (or whatever your attack hotkey may be) and left click your creep. Such as regular farming, denying will come with time and practice. There are other ways to deny more indirectly such as forcing your enemy to stay back in his lane by being super aggresive or some spells such as death pact (satanic item skill and Clinkz's ultimate) and dark ritual.