(covers information from an alternate timeline)
The bio-temporal chamber was a device invented by The Doctor in 2379 in an alternate timeline, for the purpose of exposing Kes, who was reaching the end of her life, to a biotemporal field, which would theoretically push her cells to an earlier stage of entropic decay, thereby extending her life.
The treatment had an unexpected side-effect, however, when it reactivated in Kes's cells dormant chroniton particles to which she had been exposed in 2374, due to an encounter with a Krenim chroniton torpedo. This had the effect of causing Kes to jump backwards in time at random intervals, essentially living her life in reverse. When Kes arrived in 2373, and explained what was happening to the crew of Voyager, The Doctor used a bio-temporal chamber to expose Kes to a precisely modulated field of antichronitons in an attempt to purge her system of the chronitons, and bring her back into temporal sync with the rest of the crew. When Kes was exposed to the antichronitons, she experienced a series of jumps, all the way back to her conception, after which she returned to 2373, regaining all of her memories of her life up to that point. (VOY: "Before and After")