A magno-grab remote was, as the name implied, a remote control for a magno-grab. Gregor Van Baalen used one to generate a magnetic hobble-field that once crippled the TARDIS. The Eleventh Doctor then stole it off the salvage merchant, and later sent it through a time rift to his earlier self, whereupon Clara picked it up and burned a message the future Doctor had carved into it onto her hand. This message, "BIG FRIENDLY BUTTON", subsequently became the clue that helped the Doctor understand how to use the crack in time to save the TARDIS. Clara had just asked the Doctor whether there was some "big friendly button" that he could push to make the emergency stop. This message indicated that the remote was the "big friendly button".
Using Gregor's remote on board the TARDIS to counteract the signal that had been sent from Gregor's ship allowed the Doctor to turn off the hobble-field, and thus avert the entire chain of events that damaged the TARDIS. (TV: Journey to the Centre of the TARDIS)