Let's say the biker is at the top of a rather steep hill, one with a 45 degree gradient with respect to the bottom of the hill, where the hill flattens out. While he is at the top of the hill, he/she has something called gravitational potential energy, which is basically energy stored in his body and bike due to the position of both at the top of the hill. At the top of the hill, the energy is 100% potential energy, 0% kinetic energy, or energy of motion.
When he/she starts down the hill, the energy that was stored up into potential energy slowly starts converting over to kinetic energy, energy of motion. Gravity is working all the while, causing the bike and the rider to accelerate as they go down the hill. When the biker is about half-way down the hill, the split between potential and kinetic energy is 50/50.
When the biker reaches the bottom of the hill, all the potential energy has been converted over to kinetic energy; it is 0% potential and 100% kinetic energy. The biker and the rider will be at their fastest speed at the bottom of the hill due to gravity causing the two of them to accelerate down the length of the hill.