This condition always applies to a specific kind of skill check, saving throw, or attack roll. You can be Reliable in multiple different kinds of rolls at once, but more than one application of Reliable to the same kind of skill check, saving throw, or attack roll has no additional effects.
When you roll a 7 or below on the d20 for a skill check, saving throw, or attack roll that you are Reliable in, you treat the die roll as if it were an 8. This specifically also means that you cannot score a Critical Failure by rolling a natural 1.