For an overview of all skills, see List of Skill Proficiencies.
Skill proficiencies describe the character's knowledge, ability and training in certain specific areas, such as Acrobatics, Coercion or History. Skill proficiencies are tiered proficiencies, as described above, and tends to be the main way to alter ability checks.
This is done either by being called upon by the DM demanding a certain type of check, or asked by a PC when an ability check is called for, if an alternative can be used. Note that skills are separate from ability scores, meaning that even skills seemingly only used for physical purposes can be used with mental ability scores. For example, a Wisdom Athletics roll might be asked for by a PC to determine if they can find the safest route to climb up a steep cliffside.
See List of Skill Proficiencies where each skill proficiency is listed, as well as the main features they cover. Some have effects that affect passive featurs, like Brawn and carrying capacity, and some grant synergistic tool proficiencies, like Alchemy and Alchemy Supplies.
Synergistic Tools
Each time you gain a tier in a skill with a tool proficiency associated with it, you receive a tier in the associated tool. However, if more than one tool is associated with the skill, you may only choose one that increases.
For example, when gaining proficiency in the Geography skill, the character must choose between Cartographer's Tools or Navigator's Tools.