On what airsoft gun are you referring?
On an M4 its a little spinny wheel behind the fake bolt.
On an MP5 its a little slidey dealy on the side of the gun.
On an AK its a slider on the inside of the guns ejection port.
On an M14 its a little dial that you turn inside the mag well.
Hop up essentially adds back spin to your BB allowing to to travel further.
"Its only a little bit on fire"