An earnest money deposit is the money a buyer deposits at contract signing, usually refundable until the due diligence period ends.
The deposit shows the seller that the buyer is serious. During the due diligence period it is generally refundable if the buyer terminates under the contract's terms, and after that period the deposit typically becomes at risk if the buyer defaults. The amount and timing are negotiated, and the deposit is usually held by a title company or escrow agent. Know the exact dates and conditions in your purchase agreement, since missing a deadline can cost you the deposit.