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Almost every wave you surf started as wind. Wind blowing over open water drags the surface, the surface wrinkles into small waves, and the wrinkles grow as long as the wind keeps pushing them. Three things set how big they get:
Drawing: A storm builds waves over its fetch. They leave the storm area, sort themselves by speed and arrive as swell.
Inside the storm the sea is a mess of waves of every size. Once the waves leave, they sort themselves. Long waves travel faster than short ones, so after a few hundred kilometres they have separated into clean, evenly spaced lines. That is swell, and the spacing in time between two crests is the period.
Energy in a swell travels at about 2.8 km/h for every second of period. A 14 s swell covers roughly 940 km a day, an 8 s swell about 540 km. So a deep low 3,500 km out in the North Atlantic reaches Portugal in around 3 to 4 days if the swell is long, and not at all if it is short and weak.
Both arrive together on most days. The app lists them separately: the first swell, a second swell when there is one, and wind sea, the short chop made by the wind blowing right now.
In the app: On a spot page, "swell 1.8 m @ 8 s from NNW" is the size, the period and the direction the swell comes from. The arrow points the way it travels.
Swell spreads out and loses height as it travels, but long-period swell loses it slowly. That is why a storm off Newfoundland can send ten-second-plus swell to the other side of the Atlantic, while a gale just offshore sends waves that die within a day.