A false 9 is a centre-forward who drops deep into midfield instead of leading the line. The movement pulls central defenders out of position: if they follow, they leave space behind; if they hold, the false 9 is free to receive the ball and turn.
The idea is older than the label. Matthias Sindelar played it for the Austrian Wunderteam in the early 1930s, and Nándor Hidegkuti used it to pull England apart in Hungary's 6-3 win at Wembley in 1953, the first time England had lost at home to a side from continental Europe.
Pep Guardiola's version is the one most fans picture. In a 2009 Clasico at the Bernabeu he moved Lionel Messi from the right wing into the centre as a false 9, and Barcelona won 6-2, with Messi drifting all night into the gap between Real Madrid's midfield and defence.
The role only works with the right player. It needs a forward comfortable receiving with their back to goal and passing quickly, and midfielders willing to run beyond them into the space it opens. Without those runs, a false 9 just leaves a team with nobody in the box.