Charles Spurgeon Commentary Exodus 25:21-22

"And thou shalt put the mercy-seat above upon the ark; and in the ark thou shalt put the testimony that I shall give thee. And there I will meet with thee, and I will commune with thee from above the mercy-seat, from between the two cherubim which are upon the ark of the testimony, of all things which I will give thee in commandment unto the children of Israel." — Exodus 25:21-22 (ASV)

It was the meeting place of God and people, where the law was covered with a solid plate of gold, so is Jesus the meeting place between God and sinners, where the law is covered with his perfect righteousness.