Meiosis is the cellular division that creates sex cells (sperm and egg) in organisms. This is in contrast to mitosis, which creates somatic (body) cells during growth and repair in multicellular organisms.
DNA is only replicated during Prophase I of Meiosis. DNA does not undergo another replication during Meiosis II. Therefore, Meiosis I is called a reduction division because is results in the reduction of the diploid number of chromosomes being split into the haploid number.
The diploid number of chromosomes refers to the normal number of chromosomes that is characteristic to all somatic cells of a species (di = 2). The diploid number is often represented by a (2n). The haploid number refers to half of the diploid number. Sex cells contain the haploid number (represented by just an "n") of chromosomes.
For example, somatic cells of a normal human cell contain 46 chromosomes whereas a sex cell contains 23 chromosomes.
This reduction division is needed so that when the egg and sperm unite during fertilization, the diploid number of the species is restored.
Meiosis I is preceded by the period known as interphase which is actually when DNA is replicated and not during prophase I as the previous answer stated. There are three steps in interphase-- during G1 of interphase, the cell undergoes a period of growth. During S phase of interphase, DNA is replicated in order to prepare for cell division later on. Finally, during G2 of interphase, the cell undergoes another period of growth and centrioles are replicated. As the cell cycle continues, the diploid cell undergoes meiosis, a reduction division that ultimately results in the formation of four haploid cells called gametes.