40 lines (also called Sprint, Time Attack, etc) is a game mode where you play the game until you clear 40 or more lines.
In most games score does not have any effect on this mode's ranking. Instead, time is most important element in this mode.
Some games allow players to choose level. And a few games allow players to choose the height of playfield or amount of initial garbage. In most games the level does not increase throughout the game.
In some games, the "lines" does not mean the true "lines" you cleared. Those games have "bonus lines" for multiple line clears.