I think it can have different meaning to different people and in different situations. One love that`s surely "Forbidden" is adulterous love affair between married people. Another kind of "forbidden love" would be two young sweethearts loving each other against their parents wishes.
This could be where two people love each other but is morally, ethnically or legally wrong. Such as brother and sister in a sexual way, inter caste, or historically in religion.
When you love someone you can 't have. Like, if you've loved someone for a long way but they don't love you back or don't know you love them and have another partner. There's also a forbidden crush, which is similar.
Forbidden love is tough love. it is a challenge to someone who experienced it. it also means love between two people of the same sex, between man and man and woman with woman......