。
She was sitting in a chair reading a magazine.
Literal
She [topic-は] chair [in-に] sat [and-て] magazine [object-を] was-reading.
椅子に座って sets the position; 雑誌を読んでいた is the past-progressive depicting the ongoing scene. Combined, the two read as 'was sitting and reading' — Japanese frequently chains 座って + verb of activity to render English's 'sitting + V-ing.' 雑誌 is the standard noun for a magazine; Japan retains a robust magazine-publishing culture, with weekly and monthly periodicals covering everything from manga to literary fiction to specialty hobbies.