Recombination is an event, occurring by the crossing-over of chromosomes during meiosis, in which DNA is exchanged between a pair of chromosomes. Thus two genes that were previously unlinked, being on separate chromosomes, can become linked because of recombination; and vice versa: linked genes may become unlinked.
Like mutation, recombination is an important source of new variation for natural selection to work upon. However, also like mutation, recombination places a genetic load upon the population.
As David Haig explains, the reason for recombination has been a puzzle for evolutionary biologists.