This means that the spark plugs fire in that sequence, starting with cylinder #1 and ending with cylinder #4.
Here's how to remember it:
* 1-5-3-6-2-4 - Just memorize this sequence.
* Think of it as two groups: 1-5-3 and 6-2-4.
This firing order ensures a smooth and balanced engine operation by staggering the power strokes of the cylinders.