https://www.gamespot.com/articles/destiny-2-psa-dont-spend-all-your-upgrade-points-i/1100-6453146/

One of the many ways in which Destiny 2 makes small but welcome changes to the original game concerns subclass upgrades. No longer do you obtain new abilities and skills simply by grinding away; something called Upgrade Points are now used to pick out precisely the things you want to unlock. But just because you have options for things to spend your Upgrade Points on doesn't necessarily mean you should do so right away.