Forbidden love applies whenever an obstacle, such as parents, social norms or religion, tries to prevent two individuals from being together. This can result in a secret relationship.
This trope is a classic, combining drama and angst with desperation and passion. The love interests are separated by fate or destiny, desperately trying to get together and fighting the stacking odds against them. It has existed for an eternity (think Romeo and Juliet), but I have not read a lot of books featuring forbidden love. Maybe that’s because I enjoy it when the two love interests share many scenes, and with this trope, sometimes that just doesn’t happen.
For forbidden love tropes, check out: The Demon King by Cinda Chima Williams and An Enchantment of Ravens by Margaret Rogerson.