Skill Checks are quite similar to stat checks, but with these checks you can use the level you have in your level as a modifier. You still have to roll against originally against the stat, yet skill level acts as modifier. If the action requires 2 stats, then you roll against the avg of the 2 stats.
For example… you’re trying to find a certain individual in a huge city. The GM asks for a Mind Stat Check, and you have level 2 in the tracking skill. You have a Mind Stat of 7, but with the 2 in tracking, you now only have to roll a 9.
You can perform actions that you don’t have skills in (i.e. tracking a person, hiding) but you won’t be able to add in the modifier, plus it’s more likely the GM will add in a difficulty modifier.
Almost every time you do a skill check, you’ll be rolling against the stat used for the skill. Another important fact is that the max skill modifier you can use with a roll in battle is 3. Lastly, the level cap for every skill is 6.