Ploidy is the number of complete chromosomes sets in a biological cell.
Haplo means single, just as mono does, but haploid is specific to the gametes bearing 1/2 the genetic material while monoploid is the basic ploidy number. Bacteria are generally monoploid. The basic unique set of chromosomes in the monoploid or the gamete make up the genome of the organism.
Diploids have two complete homologous sets of chromosomes. One copy comes from each parent in sexual reproduction. In order for a diploid to donate a single copy the diploid must generate haploid gametes with a single set of chromosomes.
Not all organisms are diploid for their entire life cycle and some are never diploid. Plants have an alternation of life stages between the haploid gametophyte and diploid sporophyte. One set of chromosomes in the gametophyte is enough for life to proceed; two copies just ensure a noncorrupted backup copy of the genome and permits genetic recombination in sexual reproduction.