This is the 306th most frequent Swedish word.
viktiga
Important.
Here, 'viktiga' describes 'detaljerna' (the details) as important, functioning as an adjective in its definite plural form in agreement with the noun.
De viktiga detaljerna måste inte glömmas.
The important details must not be forgotten.
In this sentence, 'viktiga' is used combined with 'mest' (most) to add emphasis, modifying 'målen' (the goals) in a comparative sense.
Ni måste fokusera på de mest viktiga målen.
You must focus on the most important goals.
Here, 'viktiga' modifies 'reglerna' (the rules) to convey that these rules are significant, showing its descriptive role in a specific context.