Deny

Denying allows players to reduce their opponents’ gold and experience income by killing their own allies or structures at low health.

When it comes to experience, denying varies depending on if your enemies are ranged or melee. If your opponent is melee, they gain 66% of the experience, while if they are ranged they get 33%.
You can deny your own creeps by letting them to die to neutrals, which will deny your opponents the full experience.
You can deny allied creeps below ~50% health and allied heroes who are affected by certain DoTs.
Denying an allied hero reduces the gold and experience given to 66%.
Towers and other structures can be denied once below 10% health, preventing the enemy from claiming full bounty.

Denying is an important mechanic to prevent your opponents from steaming ahead.
Denying your own creeps helps you control how much your lane is pushing.
Keep creep waves balanced to set up easier denies without pushing the lane.
Hold attack until the last possible moment to avoid telegraphing the deny.
Coordinate with allies to secure hero denies when they are ticking down from DoTs.
Use abilities that deal non-lethal damage to line up a deny without accidentally killing the target early.