This is the 506th most frequent Finnish word.
asian
The Finnish word "asian" means "the matter" or "the issue" in English.
In this sentence, 'asiasta' (from 'asian') refers to a specific subject or topic being discussed.
Hän tietää paljon asiasta.
He/she knows a lot about the matter.
Here, 'asia' is used to mean an issue or subject that holds importance.
Tämä on hyvin tärkeä asia.
This is a very important matter.
In this usage, 'asiat' (the plural form of 'asian') refers more generally to 'things' or 'matters' in a non-specific way.