A nerd is someone who spends a lot of time on unpopular things, such as computers, video games, comic books, science fiction, or school subjects like math and science. They tend to have poor social skills and may be introverted or shy. Nerds often get picked on by other kids at school for being different.
The word nerd used to have a negative connotation, but it has since changed. It is now more common for people to describe themselves as a nerd because they enjoy certain hobbies, such as reading or playing video games. Nerds also have their own culture and subcultures. They may spend time together at home or at school. They can share inside jokes and knowledge about a particular subject or TV show. Nerds can talk about the same things at work or at dinner parties and they can make new friends from similar interests.
Nerds are unpopular because they spend too much time studying or working on their hobbies. They don't have the time to attempt to be popular, which would require a lot of extracurricular activities. They are not able to compete with other kids for the attention of girls or boys. Nerds are easy targets and most of the time they do not have enough social capital to dissuade other kids from picking on them. They also do not have the social skill to make other kids respect them.
Many high school kids don't like nerds because they are not the same as them. They want to be popular themselves and they don't understand the lifestyle that a nerd leads. Nerds are not losers. They are simply playing a different game that is more like real life than the artificial one played at school.