Original: Dumbledore, who was dying from a slow-spreading curse, wanted to protect Snape's position within the Death Eaters and spare Draco from completing Voldemort's task to murder him.
Simplified: Dumbledore, who was dying from a slow-spreading curse, wanted to protect Snape's position within the Death Eaters and spare Draco Malfoy from completing Voldemort's task to murder him.