If you have just the definite noun on its own, "skjørtet", the definiteness is signified by the definite suffix (-et) alone.
"Hun har skjørtet."
"She has the skirt."
When the definite noun is preceded by an adjective, that noun still carries the definite suffix, but "det/den" is added as an additional marker of definiteness.
"Hun har det gule skjørtet."
"She has the(/that) yellow skirt".
This is not done when the adjective modifying the noun is placed after the noun (predicatively), so I assume that the reason we do it when it's placed before the noun (attributively), is because the definite suffix ends up so far back, that we think it important to announce the noun's definiteness earlier in the sentence.
"Skjørtet er gult."
"The skirt is yellow."
Mark that adding a determiner to a definite noun not modified by an adjective, changes the meaning from "the" to "that".
"Hun har det skjørtet."
"She has that skirt."