Runoff is the passive version of quantitative-tightening. Rather than selling bonds, the Fed simply stops rolling maturities beyond a monthly cap. Anything above the cap is reinvested, which keeps the pace predictable.
Every dollar that runs off destroys a dollar of bank-reserves or drains the overnight-reverse-repo-facility. That is why runoff eventually collides with money-market plumbing, and why the pace is normally slowed well before reserves become scarce.
Example: the cap is $25 billion a month for Treasuries. In a month with $40 billion maturing, $25 billion runs off and $15 billion is reinvested. Over a year that removes roughly $300 billion from the balance sheet.
Related: quantitative-tightening, bank-reserves, standing-repo-facility, overnight-reverse-repo-facility, tapering