The “dominant wave period” is either the swell period or wind-wave period – whichever is more pronounced. The swell period is the peak period in seconds of the swells. If more than one swell is present, this is the period of the swell containing the maximum energy. The wind-wave period is the peak period in seconds of the wind-waves.
In simpler terms, it’s how often the waves go up and down. The longer the period, the more time there is between the crests of the swells.