What is the difference between 'rocky soil' and 'fine soil' in Jesus illustration?

"On that day Jesus, having left the house, was sitting by the sea; 2 and great crowds gathered to him, so that he went aboard a boat and sat down, and all the crowd was standing on the beach. 3 Then he told them many things by illustrations, saying: “Look! A sower went out to sow; 4 and as he was sowing, some [seeds] fell alongside the road, and the birds came and ate them up. 5 Others fell upon the rocky places where they did not have much soil, and at once they sprang up because of not having depth of soil. 6 But when the sun rose they were scorched, and because of not having root they withered. 7 Others, too, fell among the thorns, and the thorns came up and choked them. 8 Still others fell upon the fine soil and they began to yield fruit, this one a hundredfold, that one sixty, the other thirty. 9 Let him that has ears listen.” Mt13:1