Sandbagging refers to withholding information on pending deals, or perhaps even slowing down the completion (a.k.a. close) of deals, in order to best meet your individual targets and/or to get rewards.
One reason to sandbag is to avoid being assigned an overly aggressive sales quota. Once a salesperson overachieves in a particular quarter, they are often 'rewarded' with a new, higher quota for subsequent fiscal quarters. This may be an unwanted challenge, so holding back a deal until the start of the next period can prevent unsightly overachievement, as well as reducing the stress of making the next quarter's numbers.
There may also be an upcoming spiff which offers extra incentive to the salesperson for closing the deal during a specific period of time. If the incentive program is too predictable, the sales staff will plan for the spiff and time deals accordingly.
Depending on their MBOs, product managers may encourage sandbagging, if for example they have a tough Q3 goal and they want to defer some Q2 deals into Q3.
This is generally undesirable behavior for the company itself. Even there, however, it can be useful, if it allows the company to underestimate upcoming revenues and then surge at the last moment to 'beat the street.'