Alternation of generation is a type of life cycle seen in some species of plants; it has no comparison among mammals. Here is a very rough analogy: rather than you and your partner each giving half of your genetic information to your child via sexual reproduction, you create a “half-clone” of yourself, which can then clone itself multiple times until you have a small army of clones, each of which could then go off to meet up with someone else’s half-clone and then fuse into an actual child.
Sexual reproduction involves a process called meiosis, in which reproductive cells (sperm and eggs) are formed by cutting out approximately half of the genetic information found in the parent cell. This allows the other half of the genetic information to be provided by the sexual partner, resulting in offspring that are a unique combination of both parents’ DNA. The adult form of the organism has n number of chromosomes; the sex cells (sperm and eggs) have n/2 chromosomes.
In the alternation of generations life cycle, rather than simply fusing one sperm cell with one egg cell, the organism goes ahead with the creation of cell that have n/2 chromosomes, but these cells then begin to grow and divide utilizing the normal growth process (mitosis). The resulting body, the gametophyte, is then strictly responsible for the eventual production of actual reproductive cells, or gametes, which go on to unite with another gamete and form offspring.
This is a relatively rare process and is found in only a few groups of organisms, such as red algae.