It's hard to be very precise and specific, because different facilities use the term "float pool" in different ways, but, generally
, a "float pool" is a group of staff that work on whatever unit they're needed on any particular day (they "float" throughout the facility).
In some places, "float pool" is the same as working "prn" or "per diem" -- you are not guaranteed any particular number of days/hours per pay period; they just let you know when they need you, and you let them know whether or not you're available (want to work) on those particular days.
In other facilities, "float pool" staff (nurses and CNAs) work a full- or part-time position with a specific, guaranteed number of hours per pay period, but they just don't know on what unit
they're going to be working those hours.
Some places require you to work some specific number of hours and/or holidays per year in order to remain in the float pool or on the prn list.
You would have to speak to a specific facility to find out exactly how they
define "float pool" and what the specific requirements would be (and you would definitely want to be v. clear about the details before you agreed to take the job). It's a way for facilities to have some flexibility and "wiggle room" in meeting their staffing needs. Like everything else in nursing, some nurses really like working as a "float," and others would never want to do it under any circumstances -- the great thing about nursing is that there's something for everyone!