What is certified cost or pricing data, and when does the government require it?

My company is bidding on a federal construction project, NAICS 236220, for a new administrative building. It's a full and open competition. The RFP mentions certified cost or pricing data and I'm trying to figure out what that means for our proposal. We usually do fixed-price contracts for state and county work, so this is new. I've heard it has something to do with contracts over a certain dollar amount. What exactly is certified cost or pricing data, and when does the government require it?