Swell is the collection of waves moving away from a storm in the ocean. Although the waves will all be of different size and power and heading in slightly different directions we can tend to talk about averages of all these waves as one discreet swell. This swell as it heads into shallow water at your local beach will make the waves you surf (or not depending on it's size and direction!).
MSW seperates the ocean at any one time into a maximum of three swells. Our 'primary' swell is the one we think is likely to make the largest and most powerful waves on the beach and is what our ratings and 'surf' heights are based on.
The swell height we give is an average of the largest 1/3rd of all waves. Something very much like the average set wave. It's measured from the trough (very lowest point) to peak (very highest point) of each wave.
Generally speaking the larger the swell the larger the waves it'll create. While this should be fairly obvious it's complicated because longer period swells also make larger waves in shallow water and the direction of the swell will also have an effect so these factors are important too.
Swell period is literally the time it takes for successive waves to pass the same point in seconds. Practically the peak period of a swell gives a great idea of how powerful the swell is and how likely it is to create good waves for surfing. Longer period swells also have a flatter profile in deep water, so create larger waves when entering shallow water to create surf.
4ft @ 10 seconds = 6ft breaking waves
4ft @ 20 seconds = 9ft breaking waves
Doubling the period gives about a 50% increase in the height of the breaking waves from the same sized swell.
We have an easy guide to what sort of waves to expect at different periods here.
To make waves on your local beach the swell needs to be heading towards it. Simply put the more the swell arrow is pointing straight towards the beach past any obstructions the larger the waves will be all other factors being equal. For even the most exposed locations once the swell moves beyond about 90 degrees from this optimum location the chance of any waves at all rapidly diminish. It's quite possible for a very large swell to travel along the coast straight past the beach without making ridable surf.
There are some exceptions, longer period swells refract (bend) better around obstacles and the precise shape of the sea bed and any other local obstructions can alter the wave size. None the less swell direction is absolutely crucial and 10-20 degrees difference can have a massive effect at some locations.
In all our experience with novice forecasters swell direction is one of the most overlooked factors when trying to read the forecast.