Lara gains experience as she collects rare items, defeats enemies, completes missions and challenges, and progresses through the story. This experience periodically awards her with skill points. Those are spent at Base Camp to purchase new skills. All of these make it easier for Lara to survive, but there are different ways for her to invest these points.
There are three types of skills (Brawler, Hunter, and Survivor). Each type of skill has three tiers. You can’t invest in the second or third tier of skills until you’ve already purchased a set number of lower tier items. This ensures that Lara has to wait for the stronger skills until she’s gotten further into the game. However, you do NOT need to invest in any specific type of skill to unlock the higher tiers of the others. Thus, you can put points into Brawler, Hunter, and Survivor skills and still unlock the second tier for all of them just as quickly as if you invest everything in one or two of these lines.