Plain English Meaning
You went past the line you were aiming for. While turning onto final, the airplane drifted beyond the runway's centerline instead of rolling out neatly on it.
Full Definition
In a turn to final approach, overshoot is the condition in which the airplane passes through the extended runway centerline because the bank angle was too shallow or the turn was started too late. It is the situation that often tempts a pilot to tighten the turn with rudder, which can lead to a cross-control stall.
Context Anchor
Seen in traffic pattern discussions, especially when a pilot turns from base to final and goes past the .