People use manure to fertilize gardens and fields. Salt without taste is so useless it is not even worth mixing with manure. Alternate translation: "compost heap" or "fertilizer"
Jesus is saying that what he has just said is important, and people may need to take some effort to understand it. See how you translated this in Luke 8:8. Alternate translation: “Let the one who has ears to hear listen” or “If anyone can hear, let him listen and understand” or “If you have ears to hear, then listen"