。
She fastened a belt around her waist.
Literal
She [topic-は] waist [at-に] belt [object-を] tightened.
締める ('to tighten, fasten, tie') is the active verb for the moment of putting on a belt, tying a knot, or fastening a seatbelt. It complements the resulting-state ベルトをしている ('be wearing a belt') — 締める marks the act, ~をしている marks the state. The same verb extends figuratively: 気を引き締める ('brace oneself, focus'), 財布の紐を締める ('tighten one's purse strings'). Whenever Japanese needs the image of pulling something snug or pulling oneself together, 締める is on hand.