So this is how you adjust the bridge saddles on a Fender bass guitar, and that's actually setting the action too. That's another way to say it. What you want to do is you want to take a measurement. You do that by pressing on the string of the first fret and measuring at the twelfth fret. I like to measure in 64ths. On the bass side, the low E, I like to get about six 64ths, and on the treble side which is the G, about five 64ths.
After you measure it, what you want to do is you do the two outside strings first, the E and the G string. If you need to lower it, you just adjust these screws here, putting them up or down. Take a measurement again. Don't forget to always press the string down at the first fret to get the measurement. That's a little low so I'm going to raise it up. Now it's a little high. Now we're going to get the G string which we want to be five 64ths. That's a little low. Let me get the measurement, and that's perfect.
Once you have those two outside strings, you're going to take a radius gauge (looks like this, made by Stewart MacDonald) and the first thing you want to do is figure out what the radius of the neck is because you want to match that. This one is going to be, like most vintage Fender basses, this is going to be seven and a quarter. That's seven and a quarter inch radius.
Then what you want to do is put it across the strings near the saddles. You want the strings to match that radius. Looks like this one, the A string and the D string, are a little bit low. You want to keep adjusting them until they're all perfectly matched. The reason you want the radius to match is because it makes it more comfortable to play it if's in line with the radius of the neck. That's how you adjust the action on a bass guitar.