GP meant "reducing balance sheet". When the fed buys long term bonds it issues short term debt thus increasing its balance sheet and incidentally reducing long term rates. During the great financial crisis, the fed bought a lot of long term debt increasing its balance sheet to levels not seen since World War 2. It is currently in the process of reducing its balance sheet which should have the effect of increasing long term rates, but they aren't responding as much as predicted. This is why some people fear a recession.

There are many other factors, but it should be noted that this is the longest time of a financial expansion (time since the last recession) in modern history. In the 70s and early 80s there were 4 recessions in a 12 year period.