Luke 8:8, 13

Yet some of the seeds fell into good, fertile soil, and they grew and flourished until they produced more than a hundredfold harvest, a bumper crop.” Then Jesus added, shouting out to all who would hear, “Listen with your heart and you will understand!”

The seeds falling on gravel represents those who initially respond to the word with joy, but soon afterward, when a season of difficulty and harassment of the enemy comes to them, they wither and fall away, for they have no root in the truth and their faith is temporary.