Hypercubes were a communication technology used by the Time Lords which resembled cubes; they were used by thinking or speaking to them and then sent through time and space to Gallifrey or to someone they trusted enough to rescue them. They could be used by humans as well as Time Lords.
“It’s a very special sort of box. It now contains all the information about what’s been going on here, and an appeal for help.” — Second Doctor, The War Games
“Time Lord emergency messaging system. In an emergency, we’d wrap up our thoughts in psychic containers and send them through time and space.” — Eleventh Doctor, The Doctor’s Wife
Part of a series on the #Tech of Doctor Who