The franchise started life with the 1986 cult classic movie Highlander, starring Christopher Lambert as Connor MacLeod, a 400-year-old Immortal, one of a number of humans with the ability to live forever. Immortals increase their power, skill and strength by beheading other Immortals, with the idea that eventually there will only be one Immortal left wielding immense power.
Four additional Highlander films followed, with Highlander II: The Quickening (1991) particularly reviled for trying to retcon the origins of the Immortals as aliens. The additional films were not well-received, but a six-season TV spin-off, Highlander: The Series, ran between 1992 and 1998 and attracted critical acclaim and a cult following. The series featured Adrian Paul as Connor's relative Duncan MacLeod pursuing his own destiny.
It is assumed the new project will be a reboot, but it's unclear if Cavill will be playing Connor MacLeod or a new character. The project is envisaged as a movie with sequel potential, and also the possibility of a spin-off TV series to expand the mythology.
Cavill is currently starring in The Witcher for Netflix and recently agreed to reprise his role as Sherlock Holmes for the sequel to the successful Netflix movie Enola Holmes. Cavill is also being rumoured to be returning in his role as Superman in the DC movie universe, and is involved in a new Mass Effect project, although whether that's voicing a character in the upcoming fifth game in the series or working on the long-rumoured live-action adaptation is unclear.