A single line clear is the most simple form of game progress in Tetris. By assembling multiple blocks (commonly called tetriminos, or minos) in a way that fills out one horizontal line of the grid, the line disappears and is thereby "cleared".
While the single line clear is a modest, but surefire way to score points in single player tetris, it is considered an undesirable move in multiplayer Tetris. That is because a single line clear usually does not send your opponents any line (garbage) at all, unless it's part of a combo.
The minimum amount of lines you have to clear at once to send any garbage is two. Therefore, a single line clear is considered a "waste", because you are just clearing a very small portion of your own grid, without putting pressure on your opponent(s) through sending them lines of garbage.
In essence, the player using a single line clear in multiplayer Tetris will seem either clueless or desperate: An inexperienced player might use it simply due to being unaware of the disadvantage vis-a-vis clearing two or more lines at once. Meanwhile, an experienced player is only likely to use single line clears when their grid is almost full, and they are in desperate need to quickly clear any amount of lines they can - ergo, close to losing the match.
In real life, the single line clear in multiplayer Tetris might be compared to a displacement activity: You are under pressure or overwhelmed by the situation, and therefore start doing something that does not constitute progress in terms of the challenge at hand (in this case, beating the opposing player), but rather buys your brain time until the threat is gone (the match is over), or you have figured out an actually effective action.