In the US, "title" is the document that shows ownership of the car. It is a nicely printed document you get from the DMV, that includes the information about the car and about you.
You "sign off the title" when you sell the car - part of the title is a form on which the owner of the title can assign it to someone else. With your signature on the title, the new owner goes to the DMV which exchanges it to a new title in the new owner's name. Never sign on the title unless you got the payment for the car from the buyer.
Usually, when the car is bought with a loan, the lender holds the title. Since you need to sign off the title to pass the ownership if you sell the car - lender holding on to it will prevent you from selling the car until the lender gives you the title back (when you pay off the loan).
Your boss, acting as a lender, wants the title to hold on to it to prevent you from selling the car that secures your debt to him. He wants that (usually pink) piece of paper. Here's an article explaining about the title and showing a sample.
Lenders holding the title will usually also add an endorsement at the DMV, so that you can't go and claim that you lost it.