A changeling is a human with faerie blood who has not given in fully to their fey heritage. The bloodline could come from a parent who is of a fey race, or it could be genetic throwback due to a recessive gene originating from an ancestor.
They might exhibit traits or powers of their faerie race, such as Martin Capulet and his enhanced strength (from his goblin father), but their racial ties are still firmly rooted in the mortal world. Accepting their fey heritage would switch that balance to the faerie side, meaning that the changeling would come to resemble those of their fey lineage and also gain more of their racial abilities and powers. But the person would also be drawn to spend more of their time in the Nevernever and less in the mortal world. Over time, they would lose some of their humanity as the fey side becomes dominant, in terms of both physical form and personality. It’s knowledge of this transformation that keeps many Changelings hanging onto their mortal lives rather than embracing their otherworldly heritages.
Thus far in the campaign, we have seen Changelings who were part goblin (Martin Capulet), part elf (Dania Lansey) — both from episodes 2.21 and 2.23. And someone who is at least part leprechaun (Charles B. Smith).