A young, attractive woman married to an older, more powerful man.
His role in the relationship is to be her sugar daddy and provide her with power and material wealth. Hers, beyond providing sex, is to remind others that he is powerful or rich enough to be desirable to such a woman despite his age and thus to serve as a marker of this status -- hence the "trophy" part. A specialized type of gold digger.
My asshole ex-husband went out and got himself a trophy wife as soon as he made partner.