Cher once expressed the wish that she "could turn back time" in her hit song. Well, in 2 Kings, God can—and does—turn back time, thus one-upping Cher. When King Hezekiah is dying, he begs God to extend his life. God agrees, adding fifteen years onto his life (as a form of appreciation for Hezekiah's righteousness). He seals the deal by making the shadow on the sundial move back ten degrees, effectively reversing the movement of the sun itself across the sky (or reversing the rotation of the earth).
This is more than just an example of God's mercy. It also symbolizes that God controls time—he is both inside of history and above it, controlling it. He doesn't just determine the big historical events, like Sennacherib's invasion or the exile in Babylon, but the details of specific human lives, as in this case.
God doesn't just know what every person ate for breakfast, and even what every bird or animal ate for breakfast (if they're breakfast-eating animals)—he's planned all of those things out in advance. It's a bigger and more comprehensive idea of what a deity is capable of doing than any of the surrounding Near Eastern religions seem to have possessed. It's one of the big take-away messages of the story of how God saves Hezekiah and also the greater story of how God saves Judah from Sennacherib's invasion.