The sap is needed by Danica Pure-Spring to revive the wilting Gildergreen tree in the center of Whiterun. As a symbol of growth, by reviving the tree, Danica hoped to increase the number of pilgrims who frequented the tree and help the wounded soldiers in her care find courage.
It can be obtained from the Eldergleam Tree in Eldergleam Sanctuary. Once in Eldergleam, follow the path using Nettlebane to clear obstacles along the way. Collect the sap and return to Danica in Whiterun.