A Mind Patch was a procedure that added the knowledge and mental capacity of one or more individuals into a hologram's memory.
It was both immoral and illegal, having been known to have adverse effects for those on whom the procedure has been performed (e.g. reducing some to gibbering simpletons). The recipient of the Mind Patch had access to the knowledge of the donors, but their own personality had control.
Kryten administered a Mind Patch to Rimmer in his attempt to join the Holocrew of the Enlightenment, which gave him a mental capacity capable of matching his opponent (the crew of Red Dwarf were the intellectual equivalent of domestic science teachers). Rimmer apparently didn't have the capacity to cope with a Mind Patch and his mind unmeshed as the Mind Patch was rejected, giving Rimmer back his own meager intelligence, but otherwise with no lingering adverse effects. (RD: Holoship)
It is possible that the Rimmer seen from "Back to Earth" onwards is a hologram of the resurrected version of Rimmer who had the memories of the original implanted into him. This would technically be a Mind Patch, although it appears to have none of the legal or ethical problems of implanting as it is just the memories that are being implanted, not the whole mind and the person whose mind they're being implanted into is the same as the one who it came from originally. There were also no noticable side effects.