Esta operação tem a ver com o fim previsto para o "quantitative easing" (flexibilização quantitativa), programa (política monetária) de estímulo às economias através do qual a Fed e o BCE, por exemplo, se propuseram adquirir dívida soberana para manter os juros em baixo tendo em vista a recuperação das economias.
Esta política monetária é/foi temporária e vai/deve terminar em breve.
A frase da consulente fala precisamente disso.

"As economic conditions continue to improve, evident in the labor market and rising inflation, albeit gradual, the Fed faces growing pressure to address its balance sheet. In December, the Fed stated it would not begin the process of shrinking its balance sheet until "normalization of the level of the federal funds rate is well under way." When that will be, or more importantly, at what level Fed officials believe normalization is underway remains unknown. Putting this subjective notion aside, when the Fed does begin to reduce its balance sheet, it will do so in one of two ways. It can sell securities on its balance sheet, or it can choose not to reinvest maturing securities.

Read more: How Will the Fed Reduce its Balance Sheet? | Investopedia https://www.investopedia.com/insights/how-will-fed-reduce-ba...
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https://www.investopedia.com/terms/q/quantitative-easing.asp

Já agora, "tapering" é outro termo que aparece muito neste contexto, e que significa abrandamento do ritmo de compra desse tipo de dívida.

https://www.investopedia.com/terms/t/tapering.asp?ad=dirN&qo...

"2017 and The Balance Sheet Reduction
After nearly a decade of accommodative monetary policy that saw the Fed's balance sheet balloon to $4.5 trillion, the Fed announced that in October the unwind would begin. The tapering would start at $6 billion a month for Treasuries and $4 billion for MBS. The tapering would be capped at $30 billion for Treasuries and $20 billion for MBS, meaning that once these levels are reached additional payments would be reinvested.

https://www.ecb.europa.eu/explainers/show-me/html/app_infogr...

Mais informações:
https://www.economist.com/blogs/economist-explains/2015/03/e...

http://expresso.sapo.pt/economia/2017-09-20-Fed-inicia-emagr...

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