Meiosis in humans happens before birth in females, and constantly after puberty in males. Meiosis is the production of gametes, the cells used in sexual reproduction, and females possess nearly their full complement of partially developed eggs at birth. Males, on the other hand, don't even begin producing sperm until puberty, after which they create tens of millions to billions per day.
Meiosis is the method by which all multicellular animals create the cells for sexual reproduction. It relies on germ cells, which have paired chromosomes, in the testes or ovaries. They divide to create cells with unpaired chromosomes, thus containing only half the DNA of the parent cell. The process of meiosis is rather complex, but in the end it produces four cells with unpaired chromosomes. When this process creates sperm, all four of the resulting cells are used. When it creates eggs, only one of the resulting cells actually survives. The three others are known as polar bodies, and they disintegrate shortly after creation.
Despite the similarities in the basic mechanism of meiosis in males and females, in humans the resulting cells are vastly different. Eggs are complex cells of relatively great size that are actually visible to the naked eye, in the right circumstances. Sperm cells, on the other hand, are tiny and simple.