It has funded that purchase by tripling the amount of deposits held by banks with the Fed – what are called bank reserves. The standard reasoning is that this kind of reserve creation is inflationary. Banks are only allowed to offer checkable deposits in proportion to their reserves.
Economists view checkable deposits as a form of money because, like cash, checkable deposits make many transactions easier. In this sense, bank reserves held with the Fed are essentially licenses for banks to create a certain amount of money. By giving out more licenses, the FOMC is allowing banks to create more money.