不久 (bù jiǔ) is an adjective meaning 'not long (in time)', often used adverbially with 就 to mean 'soon after'. It focuses on a short time interval after a reference point, unlike 快 (kuài 'soon, fast') which can also mean 'quickly', and unlike 短 (duǎn 'short') which describes duration or physical length. It is more common in everyday speech than 随后 (suí hòu 'afterwards').