The height of the waves is the number most people pay attention to, but what does a 4 ft. swell actually mean? That can depend on where you are in the world, but here in the U.S. the height reported is measured from crest (top) of a wave to the trough (bottom). For you scientists (and southern hemisphere folks), that's twice the amplitude. It means that if you're standing at the bottom of the valley in between two waves peaks, that's how tall the wave coming at you will be (it also means you can walk on water, so congratulations). That doesn't mean that the surf will be 4 ft., so keep reading.
Swell turns into surf when it hits the beach. Don't stress about the math, but when the depth of the ocean is approximately 1.3 times the wave height, then the wave will start to break - which means the top will pitch forward and the wave will release it's energy. So a 6 ft. swell will start to break in eight feet of water, but a 3 ft. wave won't break until the depth is four and a half feet. That's why large waves break further out and small waves break closer in. How high that wave is when it breaks is not just dependent on the height of the wave, but also how long that wave is. (And the shape of the bottom, how steep it is, how uniform, and lots of other stuff affect the break, but once again that's a whole 'nother topic - good resource here).
The length of the wave, so important to knowing how big the surf will be, is given by the period. That's the time it takes between one crest and the next - basically how long you have in between waves. A long period (over 15 seconds) means the waves are further apart; a short period (under 10 seconds) means they will be close together. It can be nice to have a long period because it gives you more time to get in or out between the waves, but that's not what's most important about period. Period also indirectly measures the energy of a wave.
|20 ft. @ 20 sec.|
Finally, the swell forecast will also give the direction the waves are coming from. That's useful because it allows you to figure out where will be protected and what locations might be exposed. If a coastline/harbor/cove/inlet faces the south, then it's going to be protected from a NW swell, meaning the waves will be significantly smaller than the forecast (the forecast is for the open, unprotected ocean). But a SW swell would likely slam into that same coastline/harbor/cove/inlet,
All this means that knowing the swell is 3 ft. doesn't really tell you much of anything about what the surf will be like where you're going. Some 3 ft. swells might produce nothing on the beach while others might create seven foot tubes perfect for riding or five foot dumping waves that close out. If you know the period you'll have a much better sense of the energy of the waves. If you know the direction, you'll understand how the waves will interact with your shoreline. There's definitely a lot more to the topic of surf waves, but hopefully these basics will help you get more out of your ocean experiences.