This is the fusing of the nucleus of the haploid male gamete to the nucleus of the haploid female gamete to create the first cell of the new individual. This cell is diploid and is called a zygote.
The zygote will normally be a diploid cell because it receives one set of chromosomes from the female gamete and one set of chromosomes from the male gamete.
Lots of rounds of mitotic cell division then take place. Many of the new cells produced become specialised to perform particular functions and form all the body tissues of the new individual.