。
She finished reading the letter.
Literal
She [topic-は] letter [object-を] read-finished.
読み終えた uses the compound verb suffix ~終える (to finish doing) with 読む — she completed the reading. The ~終える suffix attaches to any verb's masu-stem to mark completion of that action: 読み終える (finish reading), 食べ終える (finish eating), 書き終える (finish writing).