Cisco Ramon's treadmill is a special type of treadmill that was designed and invented by Cisco Ramon at S.T.A.R. Labs, to withstand Barry Allen's superhuman speed. It's so powerful, that it can withstand speeds above mach two.
When Barry Allen was starting to have dizzy spells, Cisco Ramon introduced Barry to the treadmill. Cisco told Barry most home treadmills go about 12 mph; he told him this one had been "Cisco-ed". While Barry was running, the team discovered his glucose levels were dropping. Barry fainted and smashed into a padded wall behind the treadmill. 
Barry was showing Felicity Smoak his speed and then decided he wanted to show the team how he can run backwards. He met the wall again. After his fight with Blackout, Barry wanted to see how fast he could go and wanted to show Eobard Thawne.
When Jefferson Jackson saw the treadmill, he asked Cisco what kind it was and Cisco said it was "Cosmic".
After racing around the city to increase his speed, he hightailed it to S.T.A.R. Labs and hopped on the treadmill, while Harry and Caitlin scored his speed. Harry said that Barry, even moving at mach 2, is still no match for Zoom currently.
Behind the scenes
- The treadmill is a reference to the Cosmic Treadmill from the DC comics, created by Barry Allen to travel through time.