It's the accusative of the feminine definite article in plural. Check the table in the Tips and notes section under Basics 1 (these notes are available on the web only) or any source like this https://en.wikipedia.org/wiki/Modern_Greek_grammar#Definite_article.
How would you say "I want her skirts"? Would it be "θελω οι φουστες τις "?
Is the letter Θ pronounced differently when it is the first letter of a word (as it seems in this case, "θέλω") ?