In a typical car, the distance from each speaker to your ear varies dramatically — a door tweeter might be 18 inches away while a trunk-mounted subwoofer is 6 feet away. Without correction, the closer driver's sound arrives first, smearing imaging and altering the way frequencies combine where they overlap.
Time alignment adds a small delay to the closer drivers so that sound from every driver arrives at your ear at the same moment, regardless of physical mounting distance. Most aftermarket DSPs calculate this automatically once you enter (or measure) the distance from each driver to the listening position.
The simplest method is a tape measure: measure straight-line distance from each driver to a fixed reference point at your ear, then enter those distances into your DSP's time alignment settings, which converts the distance difference into a time delay using the speed of sound.
For a more precise result, use an impulse response measurement at the listening position with a measurement microphone — this captures actual arrival time including cabin reflections, not just straight-line geometric distance, and lets you fine-tune from the tape-measure starting point.