This is the 259th most frequent Icelandic word.
allar
"Allar" means "all" or "every" (feminine plural) in English.
The word 'allar' here is used as an adjective modifying 'stjörnur' (stars), which is feminine plural nominative.
Allar stjörnur á himninum eru bjartar.
All the stars in the sky are bright.
In this case, 'allar' modifies 'hugmyndir' (ideas), which is a feminine plural noun in the accusative case.
Við þurfum að tala um allar hugmyndir sem þú hefur.
We need to discuss all the ideas you have.
The word 'allar' in this sentence modifies 'hlutir' (items), and it reflects agreement in gender (feminine), number (plural), and case (accusative plural).