What is a red card in soccer?
A serious foul earns a red card: the player is sent off and can't be replaced, so the team plays the rest a man short. A yellow is a warning; two yellows add up to a red.
Explain further
A red card is the harshest punishment a referee can hand a player, and it reshapes the whole game. It arrives one of two ways. A "straight" red is for something serious on its own: violent conduct, spitting, denying an obvious goal-scoring opportunity by fouling, or a reckless, out-of-control tackle that endangers an opponent. The other route is the second yellow: collect two cautions in the same match and they combine into a red. Either way, the player heads for the tunnel and is gone for good.
Here is the part that stuns newcomers. Unlike American sports, where a penalized player gets swapped out, soccer gives nothing back. A team has a fixed number of substitutions, and a sent-off player cannot be replaced by any of them. So "down to ten" is literal: that side finishes the match with one fewer body on the field, a genuinely enormous handicap.
The fallout outlives the final whistle. A red card normally carries an automatic suspension of at least one match, and the banned player usually cannot even sit on the bench. Referees can review a straight red on the pitchside monitor with VAR, so a clearly wrong call can be overturned, but a second-yellow red is not reviewable. Once the card is shown and confirmed, the team is stuck playing short.