Step 3: Compare to DC
Source Player Core pg. 393
Whenever you attempt a check, you compare your result against the Difficulty Class (DC) of the check. Your check succeeds if it's equal to or greater than the DC. If you roll anything less than the DC, you fail.
Sometimes you'll know the DC and make the comparison yourself. Other times, you might not know the DC right away. Unlocking a looted comm unit would require a Computers check, but it doesn't have a specified DC—so how will you know if you succeed or fail? You call out your result to the GM and they'll let you know if it's a success, failure, or otherwise. While you might learn the exact DC through trial and error, DCs sometimes change, so asking the GM whether a check is successful is best.
Sometimes you'll know the DC and make the comparison yourself. Other times, you might not know the DC right away. Unlocking a looted comm unit would require a Computers check, but it doesn't have a specified DC—so how will you know if you succeed or fail? You call out your result to the GM and they'll let you know if it's a success, failure, or otherwise. While you might learn the exact DC through trial and error, DCs sometimes change, so asking the GM whether a check is successful is best.
Getting a DC From a Modifier
Source Player Core pg. 393
When someone attempts a check against you, you might need to defend with one of your statistics that's normally a modifier, such as your “Reflex DC.” Rather than rolling a check of your own, you need to generate a fixed DC based on your modifier. Your DC for a given statistic is 10 + the total modifier for that statistic. So if you have a +4 Reflex save, your Reflex DC is 14.