。
She expressed her feelings about nature in a poem.
Literal
She [topic-は] nature [toward-に対する] feelings [object-を] poem [in-で] expressed.
~に対する is a noun-modifying phrase meaning 'toward, regarding, with respect to' — used to mark the target of an attitude, feeling, or action. It's the attributive form of ~に対して and modifies the following noun (here 気持ち). 詩 (し) means 'poem' — a small word with deep cultural weight in Japan, where classical forms like 短歌 (tanka) and 俳句 (haiku) have been refined over more than a thousand years and modern 自由詩 (free verse) flourishes alongside them. で here marks the medium of expression — 'in / by means of' — the same instrumental で you'd use for 'write in pencil' or 'speak in Japanese.'