A Down Payment is additional money you bring to the deal, and pay before the normal monthly payments start.
Sometimes lenders avoid risk by requiring that you put in money at the beginning of a deal. Instead of the lender having all the initial risk, a down payment gives the borrower some immediate equity and an incentive to keep paying.
A down payment also reduces the effective amount being borrowed, which creates a more affordable monthly payment.
The initial down-payment may help avoid a negative equity situation during the life of the loan.