Magnet makes your drawing points snap to a candle's exact price points (its open, high, low, or close), so your lines land precisely where you mean them instead of slightly off.
Weak and Strong. Both snap to a candle's prices, with Strong pulling harder from further away.
Force it for one drag. Hold Cmd on a Mac, or Ctrl on Windows and Linux, to force snapping for a single drag even if magnet is off.
Off. Turn magnet off entirely to place points freely.