于 is a literary/formal preposition meaning 'at/in/to.' In modern Chinese, it mostly survives in set compounds: 关于 (about), 对于 (regarding), 终于 (finally), 由于 (due to), 属于 (to belong to).
grammar
于 is the classical equivalent of 在 (at/in). You'll see it in formal writing and fixed expressions, but 在 is preferred in everyday speech.
