The term "ploidy" refers to the number of complete sets of chromosomes that are found in a eukaryotic cell. In humans and many other animals, our somatic (or body) cells are diploid - meaning that each cell has two complete sets of chromosomes. We get one set of chromosomes from our mother and another from our father.
Our gametes (sperm and eggs) are haploid - meaning that each gamete has one complete set of chromosomes.
When a haploid sperm fertilizes a haploid egg, the resulting zygote is diploid. This diploid zygote then grows into a multicellular adult organism via the process of mitosis. Because mitosis produces genetically identical cells - each cell in a multicellular organism is genetically identical (with the exception of the sperm and eggs).