Skill Checks

RAW

If the rolled D20 + modifiers is higher than the DC, the check is successful.

DCs

Very easy = no roll necessary
Easy = 9 DC
Medium = 12 DC
Hard = 15 DC

COMPLICATIONS

Missing a skill check by 1 or two might still be successful, but with a complication. A complication is less severe than the cost of failure. Jumping across a pit, but twisting your ankle on the landing for example.

RETRYING

Sometimes you can spend more time (and maybe another resource?) and retry a failed check. But often, you can't.
After a PC's skill check failure, a second PC can attempt a Double or Nothing check. PC 2 explains how their approach is different AND pays the price of failure again, After which, they can attempt the check again. If this second check is successful, the check is ultimately successful, but the 2x failure price is still paid. If this second check is a failure, the cost of failure will result in additional costs above the 2x failure price already paid.
This is similar to "Pushed Rolls" in Call of Cthulhu.
Here's an example, PC1 failues to swing across the pit of quick sand; cost = falling into the quicksand. PC2 jumps into the quicksand in an attempt to pull PC1 safely to the far side. If successful, both are safely on the far side of the pit, covered up to there armpits in quicksand. If unsuccessful, both PCs are unsafely in the middle of the pit, and PC2 is up to their neck!