Performing actions involves time, gear, and skill.

Generally, if you have none or one of the three, a task is impossible.
If you have all three, you do not need to roll – it just succeeds.
If you only have two, the task might involve a Save, with time or your gear being at stake if you fail.
Environmental factors/opportunities might act as a substitute for gear or skill.

Example: Lockpicking
Lockpicking a door requires no roll if you are skilled at lockpicking, are not in a hurry and have lockpicking tools available.

Lockpicking a door might require a DEX Save if you’re in a hurry yet have the skills and tools, with the risk being not finishing the task in time. It could also involve a WIL Save if you have time and skill, yet no tools, with you trying to improvise your tools.

Lockpicking a door is impossible with only one of the three:

You could stare at a lock all day with all the time in the world, but without gear and skill, nothing is going to happen.
You could be a skilled lockpicker, but in a hurry and without tools, there’s not much you can do.
You might have lockpick tools, but without the time to figure out how to use them, there’s no way you’ll manage it without skill.