A groomer, in child protective services parlance, is an adult who engages young people in an inappropriate way, with the idea of working with them in later years, again, in inappropriate ways. Considering the age differences, what is inappropriate are usually the sexual connotations of the relationship.
Grooming is often part of media scare stories about paedophilia. The usual claim is that internet chatrooms are infested with them.
A common claim of the racist right in the UK is that South Asian gangs are grooming white girls for their own use. This is a variation on the old white slave trade stories, but at least one gang has been caught. Nonetheless, the phenomenon has been exaggerated and is promoted for racist ends.