2 Tim 2:20-21, NIV: "In a large house there are articles not only of gold and silver, but also of wood and clay; some are for noble purposes and some for ignoble. If a man cleanses himself from the latter, he will be an instrument for noble purposes, made holy, useful to the Master and prepared to do any good work." (TM)
Here's the figure: you are the house, your life is the house. It's a large house because people are complicated. You've got gold/silver things in your life, as well as wood/clay things. Go caveman for a second: gold/silver good, wood/clay bad.
So what are these things? I would say they are areas in your life. You might call it something different. Activities. Attitudes. Whatever. Anyway, in our example they're like bowls. The gold/silver ones are intended for "noble" use, i.e. for food that you're going to eat. The wood/clay ones (unsealed, this is the first century) are for trash, unclean stuff. You wouldn't take your trash can and eat out of it. By the same token, the Lord doesn't want to use trash cans for his work. So that's why we're called on to clean up our act, so that we can be used for whatever he wants to use us for.
Why's that a big deal? It's to keep ourselves, and specifically our trash, out of the mix. And yet God has chosen to use us for doing stuff. What's he to do? Get us to clean our act up.
Notice that this is not a requirement for being saved. You don't have to clean up your act before you come to Jesus. You come just as you are for that. The desire for change and the desire for Jesus are the only requirements. What this is, is a requirement for being used. For getting off the bench and into the game. For shaking things up and doing your part for the kingdom.
We all want to be able to do things for the ones we love. This is how you qualify to be able to do those type things for God.