This is the 9811th most frequent Spanish word.
añadirlas
"To add them"
Here, 'añadirlas' is used to indicate adding a set of items (represented by 'las') to a list.
Debemos añadirlas a la lista para no olvidarlas.
We must add them to the list so as not to forget them.
In this sentence, 'añadirlas' conveys the action of incorporating 'las especias' (the spices) into the stew.
Ellas compraron especias y decidieron añadirlas al guiso.
They bought spices and decided to add them to the stew.
The term 'añadirlas' here refers to including previously mentioned items into the project to enhance it.