The GSA Schedule, formally the Multiple Award Schedule, is the government's largest catalog contract. GSA negotiates prices and terms once with each holder, and after that, any federal buyer, and some state and local ones under specific programs, can purchase directly from that catalog without running its own competition. Several years ago, GSA folded what used to be a set of separate schedules, including the old IT-focused Schedule 70, into this single consolidated Schedule, so one contract can now cover a mix of products and services depending on what you offer under it.
Special item numbers, the actual shelves
Nobody offers everything on the Schedule. You offer under one or more special item numbers, SINs, which group similar products or services together, each with its own labor categories or item descriptions and its own negotiated pricing. Picking the right SINs is closer to picking the right aisle in a catalog than filling out a form. Buyers search and browse by SIN, so if your SIN choice does not match what you actually sell, agencies will not find you no matter how good your pricing is.
A hunting license, not a sale
Holding a Schedule contract does not obligate any agency to buy anything from you, ever. What it does is make you eligible to compete for the purchases that only run through Schedule holders. The main venue for that is GSA eBuy, a request-for-quote system that only registered Schedule holders can see into. An agency posts what it needs under a SIN, and only businesses holding a contract in that SIN get the chance to quote. Winning the Schedule contract is the entry fee. Every order after that is still a competition you have to answer and win on its own, and holding the contract itself comes with a standing list of duties, covered in living on the Schedule.
Who it tends to fit
GSA wants to see that you already sell what you are offering, at real prices, to real commercial customers, before it negotiates a government price against that history. A brand-new business with no sales record is a weak candidate, whatever the trade. The offer process, covered in getting on the Schedule, also runs long, commonly stretching across months, so it fits businesses that can fund that wait without needing the first Schedule sale to make payroll. If your trade rarely sells through Schedule buyers at all, a straightforward open-market solicitation or a GWAC might be a shorter path to the same revenue. See GWACs and who they are for for the IT-and-services version of the same seat-first bargain.