The main line, in chess, is the sequence of moves considered the most theoretically sound and thoroughly analyzed in a given opening or variation, serving as the central reference in opening theory.
Within any opening, many move orders are possible, but some have been deeply studied over decades by theorists and strong players. The main line is the one that holds up best under scrutiny: it represents the "correct" play for both sides, often leading to balanced but strategically rich positions. For example, in the Sicilian Defence, the Najdorf variation with 6.Bg5 is one of the most studied main lines in the world.
In practice, knowing the main line of an opening you play regularly helps you understand the fundamental ideas behind the expected play — White’s typical plan, Black’s counterplay, and which pieces need to be activated first. This prevents falling into unfavorable positions simply through lack of familiarity. Players rated between 800 and 1400 ELO benefit enormously from learning the first 10 to 15 moves of the main line in their core repertoire, without necessarily memorizing every sideline.