It's long term treasuries and also things like corporate debt and mortgage backed securities, both of which are riskier than the normal short term treasuries. The Fed "risks up" to drive down interest rates across a broader set of asset classes to supercharge borrowing/money creation.

Basically is a trickle down bailout. You subsidize Capitalist borrowing in the idea that it will spur ~job creation~ and a rising tide will lift all boats.