You'll mostly hear this in the Southern and Appalachian regions of the US. It simply means the saliva of a donkey. It's used to say something is worthless: It's not worth donkey spittle. Or more colloquially with "ain't" (this isn't standard English): It ain't worth donkey spittle.

Spittle is a word for spit. Spit is the water in your mouth that you can spray on someone. Donkey spittle is when a donkey spits.