The term 12th man applies to the fans present at the match. To motivate the team they are supporting, especially if they are on the losing side, they make so much noise, to show their support. The metaphor is derived from the fact that in football, there are only 11 members at the start of the game. Hence, the fans of the team are considered the 12th player.

Champions League Crowd
The 12th Man Further Explained
The 12th man’s influence is especially pronounced on the home ground of the team. Most of the arena is packed with its fans to create a home advantage. Supporters will stand in solidarity through cheering, singing, and clapping, building a positive environment for the players. The support of the fans creates a big impact on their performance during the game. To encourage the opposing players from losing concentration, the fans would often jeer or boo the opposing team, contributing to the excitement at the stadium.
During major competitions, such as the Europa League or the Champions League, the presence of the 12th man is deemed crucial. During the quarter-final or semi-final of the playoff game, if the team loses on the first match of the opponent’s ground, they will have the home advantage in the next match. The team could then take advantage of their fan’s motivation. The familiarity of their home stadium can pose an advantage as well to even out a tie. Therefore, playing the second half in a two-legged knockout match at their stadium is deemed a huge advantage for the team.
It has been observed in the history of football that a player can do better when competing at their ground. Plus, the presence of the 12th man is a huge advantage. If the team plays at the opponent’s ground, the number of supporters that can watch the game is not that many. Also, the team will need to get used to playing at an opponent’s stadium. However, if the team can practice their skills and strengthen their defensive tactics, regardless of whose stadium they play, they can still win the match.