Why "it is an easy thing" can be stated clearly. "It is easy to cause the shadow to move forward ten steps, because that is the normal thing for it to do" (See: [[rc://en/ta/man/translate/figs-explicit]])
This was probably a special stairway built for King Ahaz in such a way that its steps marked the hours of daylight as the sun's rays advanced along them. In this way, the staircase served to tell the time during the day. AT: "the steps that had been built for King Ahaz" (See: [[rc://en/ta/man/translate/figs-explicit]])