大器 (dàqì) is a literary noun meaning 'great talent' or 'a person of outstanding ability'. It's most often encountered in the chengyu 大器晚成 (dàqì wǎn chéng) — 'a great vessel takes long to finish', used to say that someone who achieves great things later in life. Outside of that set phrase it's rare in everyday conversation; you'll find it in formal writing, biographies, and pep talks. The word can also refer to a valuable object (like a treasure), but the talent sense is far more common.