Remember the Ark of the Covenant? It's that thing the Israelites built way back in Exodus after they escaped from slavery to contain the tablets of the Ten Commandments? They believed that God dwelled in it and they worshipped God through the Ark.
When the people finally got into the Promised Land in Israel they brought the Ark with them.
Unfortunately, worship of God kind of fell by the wayside. Remember King Saul?
David would like to bring back the days of serving God.
He tells the people that they should go to Kiriath-Jearim and bring the Ark home to Jerusalem. Everyone thinks this is a swell idea.
The trip hits a bit of a snag on the way back to town. One of the guys assisting with transportation of the Ark—Uzzah—touches the Ark and God kills him right then and there. You don't want to touch the Ark, even by accident.
David's pretty freaked out so he decides to make a pit stop and store the Ark in the house of Obed-edom for a few months. Lucky for Obed-edom, God blesses his house and no one else even gives the Ark a funny look.