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Two forecasts can both say 2 m, and one is a gentle day while the other is heavy and dangerous. The difference is the period.
Drawing: Same height, different period. In deep water the distance between crests is about 1.56 × period² metres: 76 m at 7 s, 306 m at 14 s.
The power in a wave, per metre of crest, grows with the square of the height and with the period: about 0.49 × height² × period kilowatts per metre (height in metres, period in seconds, deep water, using the average height of the biggest third of the waves).
In the app: The "Power" figure on spot pages is this number (kW/m). It is a better way to compare two days than height alone. Beach break at 1.4 m and 8 s is about 8 kW/m; 3 m at 15 s is about 66.
Long waves feel the bottom earlier, from deeper water, so they bend and wrap around headlands into corners that short swell never reaches. They also grow more as they reach the shallows.
The height on a forecast is a measure of the whole sea state: the average of the highest third of the waves. The biggest wave in a set is typically 1.5 to 2 times that. And what arrives at the beach depends on the spot: a sheltered corner shows a fraction of the open-ocean size, a spot facing the swell on a steep bottom can show more.
That is why the app shows two things: the swell in open water, and an estimate of the surf size at the beach, which uses the spot's own facing and shelter.
Waves of different periods travel together and pile up now and then, so the biggest waves come in groups, called sets, with lulls in between. A set of five waves every ten minutes is normal. Watch for two or three sets before you decide how big it is.