That means, to account for the unforeseen stoppages in play, a few minutes are added at the end of each half. Longer stretches are usually tacked on to the end of the game, typically resulting in an extra three or four minutes on top of the standard 90.
But, the extra time always reflects the interruptions that occur in a game. So, if a game sees multiple players carried off in stretchers, followed by substitutions, followed by a cheeky goalkeeper biding his time during a goal kick, you might see five or six minutes added.
If a game is particularly frenetic and chaotic and sees a crazy amount of interruptions, stoppage time still adds up proportionally. One famous example was a 2001 lower-division match between English teams Brentford FC and Bristol City, in which a string of mishaps saw 23 minutes of stoppage time added to the first half alone.