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 and often that person can be 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's used just for homosexual men. In that case, homosexual women are called "lesbian" instead.
Gay's negative connotations
Most people use "gay" only when talking about gay people's sexuality. Sometimes "gay" is used in an insulting way to mean that something is weak, stupid, or less important. For example, "That is so gay!" is used to say something looks or acts stupid. They may also use the word "queer" in the same way. This inappropriate way of using the word "gay" is also considered offensive.
Sometimes people also use this to mean something is very colorful. This is because of the original meaning of the word. Sometimes even gay people themselves use the word this way.