Pressing is when a team without the ball chases the opponent in possession, closing down passing lanes to force a mistake and win the ball back high up the pitch. Gegenpressing, German for counter-pressing, is the specific version of it: pressing immediately after losing the ball, before the opponent can settle and build an attack.
The idea is old, but the German word reached English-language football through Jürgen Klopp's Borussia Dortmund, who won the Bundesliga in 2010-11 and 2011-12 playing a version of it that flattened opponents inside their own half. Klopp described a good counter-press as the best playmaker a team can have, because winning the ball twenty metres from goal creates a better chance than any pass from midfield.
The tactic depends on structure, not effort alone. Players have to stand close enough together when possession is lost that two or three of them can surround the ball within a few seconds, cutting off the easy pass back and forcing a rushed clearance or a turnover deep in dangerous territory.
Pressing this way is exhausting, and a team that presses badly just opens gaps behind its own defence. It became one of the defining ideas of the 2010s and 2020s all the same, because a well-drilled press turns defending into the first stage of attack rather than the opposite of it.