A hydro bomb was a small but powerful explosive device used by both the United Federation of Planets and the Terran Empire, kept on starships such as the USS Discovery for an unspecified purpose. It could be remotely led to a detonation spot via a control panel which doubled as a detonator.
Sylvia Tilly attempted to explain to Michael Burnham why Discovery carried them, but got as far as "We keep them on the Discovery so we can re-" before being incapacitated by Philippa Georgiou.
If detonated in an active volcanic system, a single hydro bomb was enough to render a planet an uninhabitable blackened ball of dust within weeks, as the resulting phreatic eruption would vaporize any landmass in the blast radius and throw ash into the atmosphere; however, the planet's surviving inhabitants had a chance to escape, as the devastation was gradual. Still, the consequences of detonating a hydro bomb in a volcanic system were so horrific that to do so was considered an act of genocide.
In the mirror universe, Terran emperor Philippa Georgiou defeated the Klingons by detonating a hydro bomb in Qo'noS's volcanic vents. After Discovery brought her to 2257 of the prime universe, she attempted to do the same to the prime Qo'noS under authorization from Starfleet's admiralty, but was prevented from doing so when Sylvia Tilly, Ash Tyler, and Michael Burnham discovered what she was planning and alerted Discovery's crew, leading to Burnham stating her objections to the plan to Admiral Katrina Cornwell. Instead, prisoner of war L'Rell was given the control panel, which she used to assume leadership of the Klingon High Council and cease hostilities. (DIS: "The War Without, The War Within", "Will You Take My Hand?")