An emotional inhibitor was a device fitted in all Cybermen suits. It allowed a human brain to be installed into a suit and exist without the brain suffering a "meltdown" of overwhelming emotions of what they had become.
Mickey Smith hacked into Cybus Industries' mainframe and located the code on which all of the Cybermen's emotional inhibitors operated (6879760), then texted it to Rose Tyler's mobile phone, which the Doctor used to interface with the computer system. The code was shut down and the inhibitors turned off, resulting in all the Cybermen realising what they had become and dying from the emotions they felt. (DW: The Age of Steel)