This is the 266th most frequent Romanian word.
aceasta
"This" or "this one" (feminine singular).
Here, 'aceasta' is used as a demonstrative pronoun meaning 'this', referring to a specific object in the feminine singular form.
Aceasta este o carte foarte interesantă.
This is a very interesting book.
In this sentence, 'această' is an adjective form agreeing in gender (feminine) and case (accusative) with 'floare' (flower).
Copilul apreciază această floare.
The child appreciates this flower.
The word 'aceasta' follows the noun 'problema' (problem), strengthening the specification of the subject.