Time zombies were versions of the Eleventh Doctor, Clara Oswald, and Gregor and Tricky Van Baalen from the near future. A tear in time caused them to appear within the Doctor's TARDIS prior to their creation.
They had been exposed to the TARDIS' Eye of Harmony to the extent that their cells had liquefied and their skin had burnt; Gregor and Tricky had even been fused together and the Doctor had his hand fused to his face. Unlike appearances of "memories" from the recent past, the Doctor described these manifestations as "real".
After realising that their future could be rewritten, the Doctor escaped the Eye with Clara before they were transformed. He later sent back the message "Big Friendly Button" to a past version of himself, which prevented them all from being exposed, and thus no-one was ever transformed. (TV: Journey to the Centre of the TARDIS)
Behind the scenes
In Journey to the Centre of the TARDIS, Clara describes them as "zombie creatures"; the term "Time Zombie" is actually taken from the episode's end credits.