什么是 "earnest money"

看中一个short sale condo. 让agent下offer. 她回信说：
“Once I get your legal names (the names you wish to hold title in) and the amount you want to put up for earnest money (typically 2-4%, which will be cashed once an accepted offer is arrived and and then sit in escrow to be credited towards your closing costs) I will write the offer and email it to you.”

买过两次自住房，近十年前了，不是short sale, 从没有交过这个“earnest money”。 这是个新事物吗？