A GSA Schedule contract, formally the Multiple Award Schedule, is a standing agreement that pre-negotiates your prices and terms so any federal buyer can order from you without running its own competition. Getting on takes a lengthy proposal covering pricing and past performance, and staying on requires reporting sales and paying a small fee on them. Once you hold one, it functions as a storefront: agencies find you through GSA's eBuy and order directly.
In a sentence
"Quotes are requested via eBuy from GSA Schedule holders under SIN 541611." Only businesses that already hold a contract in that Schedule category can even see the request, let alone answer it.