Genetic drift is a variation caused accidentally in a population which does not give any survival advantage.
For eg : In a tree, a group of blue beetles live. Due to variation, some of them turn red. But red beetles cannot camouflage from the crow and are as visivle as blue ones. So here variation does not give any advantage. Suppose an elephant steps on the part where the blue beetles live, which leads to all blue beetles dying. Now red beetles are only found, variation led to evolution but without any survival advantage. :)
