Two copies of everything
Most of your cells are diploid: they carry two copies of each chromosome, one inherited from each parent. The two members of a pair are called homologous chromosomes — they carry the same genes in the same order, though they may carry different alleles of those genes. Humans have 23 pairs, for 46 chromosomes in all.
The number of complete chromosome sets a cell carries is its ploidy. A diploid cell has two sets; a haploid cell has one. Holding this distinction in mind is the key to everything that follows.
The arithmetic problem of sex
Sexual reproduction joins two cells into one. If a 46-chromosome cell fused with another 46-chromosome cell, the offspring would have 92, the next generation 184, and so on. The count would double every generation — clearly impossible. Nature's solution is to make special cells, gametes (egg and sperm), that are haploid. When two haploid gametes fuse at fertilization, the diploid number is exactly restored.
The cell division that produces haploid gametes from a diploid cell is meiosis. It is the engine of this whole track. Everything else — crossing over, linkage, recombination maps — happens inside it.
Halving is also a chance to shuffle
Meiosis does more than count. As it picks one chromosome from each homologous pair to put into a gamete, it makes those choices independently and randomly, and it physically swaps pieces between homologs. The result is that no two gametes you make are likely to be identical, and your children inherit fresh combinations of alleles that never existed in either parent. That creative shuffling is the deeper reason sex exists at all.