In Dungeons and Dragons and related systems, some actions depend on chance, with success determined by the roll of a D20. In some systems, including the earlier D&D systems, the player might be allowed to 'take 10', assuming an average and unassuming score of 10, eschewing the grandeur of a natural 20 or the horror of a natural 1 in favor of milquetoast certainty.
Taking 10 should not be confused with taking 20, which is a much more satisfying bit of trickery.