The goal of a design sprint is to take all of the existing research about a problem, unpack its essence to a diverse group, then ideate like crazy. The ideations are then funneled through the lenses of human centered design and those that survive are voted on by the team. Designers then build a low-fidelity prototype (often in just one day) that is just barely good enough to test on potential users. The results of the test, if positive, create a target for designers to hit in high-fidelity; thus fueling the Lean UX/Agile cycle.
Addendum: The design sprint has its roots in the IDEO human centered design process, which is closely linked to Stanford and the d.school there. McKinsey authored a similar type of sprint using the same conceptual roots. All of that traces back to Stanley Pollitt and the roots of user-centered design in advertising but that’s a story for another day.