This is the 15307th most frequent Russian word.
постную
"Постную" means "lean" or "meager," often referring to food prepared for fasting (without meat, dairy, etc.).
The word 'постную' here is used as an adjective describing the food as compliant with fasting rules, which typically excludes meat and dairy.
Мы готовили постную пищу на ужин.
We prepared lean food for dinner.
In this context, 'постную' modifies 'колбасу', indicating that the sausage is made in a way that conforms to fasting dietary restrictions.
Она купила постную колбасу для завтрака.
She bought lean sausage for breakfast.
Here, 'постные' is the plural form of the adjective 'постный' and describes dishes prepared according to fasting guidelines.