A British National (Overseas) is someone who has a special type of British nationality, which means they are allowed to live and work in the United Kingdom, but they are not actual citizens of the UK. They usually come from a place called Hong Kong, which used to be a British colony, but was returned to China in 1997.
When Hong Kong was returned to China, the UK government allowed people who were living there and had a connection to the UK to become British Nationals (Overseas). This means they can come and go from the UK as they please, but they do not have the same rights and privileges as a British citizen.
British Nationals (Overseas) usually have a special type of passport, which allows them to travel to the UK for up to six months at a time without needing a visa. They cannot vote in UK elections, nor can they get certain jobs that are only available to British citizens.
Recently, there has been some controversy around British Nationals (Overseas), because of changes that China has made to laws in Hong Kong. Some people are worried that these changes could endanger the rights of British Nationals (Overseas) living in Hong Kong and have called for the UK government to provide them with greater protection.