What is "denying" a creep?

Solution 1:

EXP and gold you get in DOTA 2 come from landing the killing hit on an enemy, be it a player or one of the creeps, which are the minions heading towards your base. However, you ONLY get rewards from you or an ally landing the last, killing hit on an enemy mob. It doesn't matter if you did 95% of the damage yourself, only the last hit counts.

Because of that, players have come up with "denying" creeps, towers and sometimes even heroes, which is basically landing the killing hit on YOUR own minion, to deny the enemy the killing hit and associated rewards. A successful deny reduces the gold and EXP the enemy team gets to 30% EXP and no gold reward, and additionally rewards the denying player with 20% of the normal gold reward (but no EXP reward).

The DotA 2 wiki provides additional information on denying:

https://dota2.gamepedia.com/Denying

You can deny a creep, tower or hero by issuing an attack-move command directly onto the ally you want to deny. The default keybind for this is the A key.