This term came from my friend, the gifted composer, Paul Dinletir. A Fixer is someone who gets things done. Simple as that.
Manager's fantasize that every employee will be a Fixer. Entrepreneurs need to be Fixers by necessity.
A Fixer doesn't care where the good ideas come from and they'll use them without getting jealous, defensive or insecure. No task is too big or too small. They see a gap and they fill it. They anticipate what needs to get done. If they finish their work early they'll help their teammates even if it means doing something that is outside of their skillset or below their pay-grade.
I'm sure you know a Fixer, they're recognizable because they're a pleasure to work with. It doesn't mean they're weak or that conflict won't happen with them but they can set aside their ego to get things done.