Mitosis creates two identical daughter cells that each contain the same number of chromosomes as their parent cell. In contrast, meiosis gives rise to four unique daughter cells, each of which has half the number of chromosomes as the parent cell.
Because meiosis creates cells that are destined to become gametes (or reproductive cells), this reduction in chromosome number is critical — without it, the union of two gametes during fertilization would result in offspring with twice the normal number of chromosomes!