Short answer: A false 9 is a striker who drops deep into midfield, dragging defenders out of position. Messi played it under Guardiola.
Traditional 9s are target men who stay high. False 9s drop into the 10 space, dragging center-backs forward and creating gaps. Pep Guardiola turned Messi into a false 9 in 2009 — and Barcelona scored 4-0 vs Real Madrid. Modern false 9s: Roberto Firmino, sometimes Harry Kane.