The same swell can be a fun wave at one tide and a closeout an hour later. Tide is the number that changes through the day while everything else stays the same, and at a lot of spots it is the difference between going and not.
Why tide matters at all
Tide changes the depth of water over whatever the wave breaks on: a reef, a sandbar, a cobblestone point. Change the depth and you change how and where the wave stands up and breaks. A swell that breaks perfectly at one depth can shut down or back off entirely at another. The wave did not change. The water under it did.
Low tide
At lower tide there is less water over the bottom, so waves tend to stand up steeper and break harder and more hollow. That can be exactly what a spot wants, or it can go too far: too shallow and the wave closes out, breaks onto dry sand, or exposes rock and reef. Some spots only really work on a lower tide.
High tide
At higher tide there is more water, so waves tend to break softer, fatter, or not at all as the swell rolls through without standing up. Some spots need that extra push of water to keep from closing out; others go flat and lifeless on it.
The per-spot tide window
Here is the part there is no shortcut for: the right tide is a fact you learn per spot. One break wants a low incoming, the next wants a mid tide, the next only comes alive on a push to high. There is no universal rule, which is exactly why a forecast that ignores tide leaves out half the story at a lot of breaks. Whether the tide is rising or dropping can matter too, not just the height.
How to use it
Once you know your spot's tide window, the tide chart tells you when in the day to go, and when to skip it even though the swell looks good. Pair that with the other four numbers in the how to read a surf forecast guide, and check what each break wants across the California spot directory.