Active management vs Bogleheads for taxes

I just met with an advisor at Fidelity. I currently work with another firm that I'm not happy with. Thinking of doing it solo, or switching. She talked about how they buy individual funds and tax loss harvest throughout the year (I can't remember what these funds are called), but how they're better than an index fund for tax purposes. Can someone explain this further? Does this make this kind of service better than a very simple 3 fund approach?