Gay is a word used to describe someone who is homosexual. Today, the word usually means a person who is sexually attracted to people of the same sex. Often that person is also romantically interested in people of the same sex. A gay person can be a male who likes other males or a female who likes other females.
The word "gay" can mean any homosexual person, but sometimes it is used just for homosexual men. In that case, homosexual women are called "lesbian" instead. Sometimes, people use the word "queer" to mean the same thing as "gay" when talking about sexuality.
In the early 20th century and before, "gay" used to mean 'happy' or 'carefree', but now people use it to define sexuality.