Abseiling is when you climb up to a height, and you are attached to two ropes; one is for you to hold, the other is for the instructor.  You had to climb 50 steps and you use your rope to lower yourself back down the wall, but if you let go, the instructor has got the other rope so you don't fall.  The instructor we had was called 'GOAT' Zak (GOAT meaning Greatest Of All Time!)