Your body is made of millions of cells. But you began your life as a single cell. That single cell was made when two very special cells joined together. The special cells were an egg cell and a sperm cell.
Egg cells and sperm cells. are called gametes.
Gametes are cells that are adapted for reproduction.

Chromosomes

Every cell has chromosomes in its nucleus. Chromosomes are long, thread-like structures They are made up of the genetic material, which contains information about how the cell will develop.
You have 46 chromosomes in every cell in your body. But gametes have only 23 chromosomes.

Egg cells and sperm cells

Egg cells are the female gametes. They are bigger than most other cells. They are about the size of the full stop at the end of this sentence. Egg cells need to be quite large to make space for food stores in their cytoplasm.
Sperm cells are the male gametes. They are smaller than most other cells. They have only a tiny amount of cytoplasm. They have a long tail, so that they can swim.

Fertilisation

When a sperm cell meets an egg cell, the head of the sperm cell goes into the egg cell. The nucleus of the sperm cell and the nucleus of the egg cell join together. This is called fertilisation.
The new cell that is produced is called a zygote.