When a girl is tied up in a relationship (offical boyfriend & girlfriend status) but is still however making advances towards another boy, often a close friend. Thes advances are not alway sexual - sometime mental or emotional, but still lead to confusion to what the offical status of the relationship is. A boyfriend in training is often confused, and can often be pussy-whipped into doing anything the girl desires, because more often than not, he has affections for her. The situation more than likely suits the girl as she does not have to comprimise her boyfriend - but can still have anything she desires from the 'boyfriend in training'.
Elaine: Is Sarah going out with Alex?
Danny: No, she still has a boyfriend.
Elaine: Well, when a pretty girl lies all over you, you don't complain.
Danny: He's her boyfriend in training.