What happens between meiosis i and meiosis ii that reduces the number of chromosomes?

In meiosis, there is only one replication (in meiosis I) but two divisions, and reduce the chromosome number half.

Explanation:

Meiosis occurs in germ cell and produce four haploid cell from a single diploid cell.

Meiosis I begins with diploid cell and there is DNA replication in meiosis I and, and it ends with haploid cells having chromosome number half as compared to parental cells, this is called reductional division.

Meiosis II begins after meiosis I, where two haploid parent cells ends with four haploid daughter cells, after meiosis II, containing the same number of chromosomes in each cell as in meiosis I but half DNA content because there is no DNA replication in meiosis II. In meiosis II, sister chromatids are separated into different chromosomes. This is called equational division.