。
She needs to take in the waist of her trousers a little.
Literal
She [topic-は] trousers [genitive-の] waist [object-を] a-little tighten [must-なければならない].
ズボン ('trousers / pants') is from French jupon (originally a 'petticoat' in French) — one of the early Western clothing words to enter Japanese, dating back to the 19th century. It coexists with the English-derived パンツ, which can mean either 'pants' or 'underwear' depending on context (a frequent source of confusion). 詰める is the tailoring sense here: 'take in / shorten / tighten' fabric. The same verb covers stuffing, packing, and shortening distance more broadly. ~なければならない is the standard construction for obligation ('must, have to').