Now, from October to December of this year, the Fed will let about $10 billion worth of Treasury and mortgage bonds expire each month instead of exchanging them.
They don't default, they're full faith and credit securities of the US government, or they are at least backed by the US government in the case of mortgage bonds.
But for many actors in the economy, the ten-year yield is more important and tangible, because it serves as a benchmark for everything from corporate bonds to mortgages.