This is the 10302nd most frequent Catalan word.
plenas
"Plenas" is not a standard Catalan word, but it could be a form of "plena," meaning "full" in English, in the plural/feminine context.
Here, 'plenes' refers to being filled with a content, books in this context.
Les caixes estan plenes de llibres.
The boxes are full of books.
In this sentence, 'plenes' describes the state of 'butxaques' (pockets) being filled.
Té plenes les butxaques de monedes.
Their pockets are full of coins.
'Plena' here metaphorically means a fulfilling or complete life, showing an abstract use.