Hot Universe Baryon Surveyor (HUBS) is a proposed X-ray mission that is dedicated to probing hot gas, which is theoretically postulated to permeate the circumgalactic space and the cosmic web, through means of high throughput, high resolution X-ray spectroscopy. On galaxy scales, the hot gas is thought to be a key component of galaxy ecosystems, produced by stellar or black-hole feedback processes that are presently not understood. In the cosmic web, the hot gas traces important physical processes of structure formation. Observationally, however, the hot gas turns out to be quite elusive, due to its (expected) very weak and diffuse emission in soft X-rays. HUBS aims at filling this void. In this presentation, we will describe the core scientific objectives of HUBS and the key technologies required.