Just to add a little extra detail to David's answer, the "package" that's sent to the lender is sent by the seller or their agent (almost always it's the agent) to the seller's lender. That lender must approve the short sale in order for the sale to go through.
The seller must complete a hardship letter and provide documentation showing their current financial status (tax returns, bank accounts, income, debts, assets, etc.) -- it's very similar to what a lender would require if you were buying a home. The package also includes a copy of the accepted offer and usually a net sheet showing the lender exactly how much they'll receive when escrow closes. The short sale lender reviews the package and obtains price opinions from other real estate professionals/appraisers to make sure that the property is being sold at a realistic price. After reviewing everything, the lender decides whether a short sale makes more sense than foreclosing.
Most short sale lenders do not want the sellers or their agent sending papers over piecemeal; they'd rather get everything all at the same time -- and not until an offer is accepted. So when you see a comment saying that "lender has received short sale package" or something like that, it often means that a previous offer was presented, but by the time the lender responded (it often takes 4-6-8+ weeks) the original buyer probably lost interest.
When that happens, since the short sale lender has already received and reviewed the buyer's package, it gives the seller and their agent a much better indication of whether or not (and on what terms) the short sale will be approved. So it can often mean a quicker response and less uncertainty for the next buyer.
- Rod Herman