A skill check is a check relating to use of a skill. The basic skill check = d20 + skill rank + the relevant ability modifier. Additional skill modifiers may be applicable. The DC to pass is variable with the task (see each skill's page for examples).
Rolling a 1 or a 20Edit
There is no such thing as a critical miss or critical hit with skill checks. A roll of 20 doesn't guarantee success, for instance; rolling a 20 to disable a strong trap which is DC 40, when your disable device skill is only 15 will always fail. Rolling a 1 also may never fail - such as unlocking a door with a DC20 lock, and having 20 ranks of open lock, since you can never get below a total of 21.