Hello there, thanks for your question!
A bj is short form or the acronym for Blow Job. A blow job/bj is a slang term used to describe oral sex performed on a male; stimulation to the penis and surrounding areas with a partner’s mouth, lips and tongue. It is also known as fellatio or ‘giving head.’
Here is a great piece called “Oral sex for him… but how?” by Scarleteen.com, for a more detailed discussion on blow jobs and sexuality related to oral sex.