Here is the description: https://www.registrar.ucla.edu/Registration-Classes/Enrollment-Appointments-and-Passes/Undergraduate-Student-Enrollment-Passes
You sign up for 2 courses (up to 10 units) in your first pass time, when most if not all classes should still have open spots. Then you see what’s left to add a third (and if desired a fourth) course in second pass. You get a specific time, which is randomized within your enrollment group (usually it’s about 1 day per enrollment group) for each pass (a bad time in first pass won’t get you a better one in second pass or vice versa), with freshmen at the bottom of the heap unless they are Regents scholars (AP credit doesn’t count for standing unlike at some schools). You log on at your designated time and pick from the classes/sections with open spots.
It’s slightly different at freshmen orientation, there your orientation leader draws lots amongst your group of about 8, and you log on one after another to pick all three of the first quarter’s classes. So no first/second pass, but everyone should get something acceptable (maybe the last one or two to choose will see some constraints).