The birthday is the day on which a person was born. For example, someone born on 1 January always celebrates their birthday on 1 January. The "first birthday" is one year after birth. So you are born on your "zeroth birthday".
In many countries and cultures, people celebrate their birthday. On this day, you want to do something special, or you are with friends and family and there is a special meal. Above all, a round birthday is celebrated, for example the 10th, the 20th the 30th and so on. If you celebrate your 75th birthday, you have turned three quarters of a century old.
In ancient times, people mainly celebrated the birthday of the ruler, such as the birthday of the pharaoh. For the Greeks and Romans, it was about honouring the gods who were there to protect the ruler. It took a long time before the birthdays of "normal" people were also celebrated.