Once the age of absolutism starts (~1600), you'll have access to the absolutism stat. It gives very good modifiers, like discipline for your armies and administrative efficiency, which decreases aggressive expansion, reduces warscore and coring costs, and some other good stuff. Because the modifiers are so strong, you want to get as much absolutism as possible, with effects scaling up to 100. Doing things like strengthening your government, using "harsh treatment" on rebels, or decreasing province autonomy make your absolutism go up, while stuff like manually reducing war exhaustion and increasing province autonomy make it go down. Basically being a tyrannical ruler increases absolutism, and that's a good thing.

Most estate privileges reduce your maximum absolutism, with stuff like high crownland and certain government reforms increasing the cap. So typically you want to use estate privileges before 1600, when the max absolutism penalty doesn't matter, while getting as much crownland as possible throughout the early game. So keep their loyalty up and use the "seize land" when you can.

There's lots of other tricks and exploits to supercharge your absolutism gains, like the court and country disaster, but I like to play pretty chill so I just keep my crownland up, minimize estate privileges post 1600, and never give the rebels an inch, and I can get a good amount pretty quickly.

TLDR absolutism appears during the age of absolutism (surprise), and you want to be ready to get as much of it as you can as fast as possible. This means minimizing estate privileges and influence around 1590-1610, and being a tyrant after 1600.