This means the spark plugs fire in that order:
* Cylinder 1 fires first
* Cylinder 3 fires second
* Cylinder 4 fires third
* Cylinder 2 fires last
Keep in mind that the cylinder numbering may vary depending on which side of the engine you are looking at.
If you are unsure about the specific firing order for your particular engine, consult your owner's manual or a reliable automotive repair manual.