It was a technology, mostly for businesses I believe, from before internet. https://en.wikipedia.org/wiki/T-carrier I mostly know it from a Weird Al song (All About the Pentiums -
Installed a T1 line in my house
Always at my PC, double-clickin' on my mizouse
Upgrade my system at least twice a day
I'm strictly plug-and-play, I ain't afraid of Y2K )