As you probably know the shape of waves, in a given place, is a mix of different kind of waves:

The windsea (also called wind waves) is produced by the local wind.

The main swell is produced by a strong wind far away.

Secondary swells are produced also by winds far away but in other places.

All of them have different heights, come from different directions and show different periods (which is the time between 2 wave crests)
Swell 1 is the main swell, swell 2 and 3 are secondary swells (the swell ranking 1, 2, 3 depends on their respective height).

See also:
https://community.windy.com/topic/3361/description-of-weather-overlays

For example close to NewZealand coast you can observe different swell trains due to several storms at this season.

In this example, main swell from the East and Swell 2 from the South are caused by 2 different storms.