Shorting a bond requires borrowing it, and the way to borrow it is to lend cash against it in reverse repo. When many traders want the same bond, they compete by accepting lower and lower rates on the cash they lend. That bond goes special.
Specialness is a direct read on short positioning. The on-the-run 10-year is frequently special because it is the standard hedging instrument. Extreme specialness can mean a squeeze is building.
Example: general-collateral trades at 5.33% and a particular note trades at 3.10% in repo, 223 bp special. Owners of that note earn a large funding advantage simply by lending it out overnight.
Related: general-collateral, repo, on-the-run, short-selling