Normally I'd just suggest you looked it up in a dictionary, however there is an interesting wrinkle here:
Wiktionary: Down payment
A payment representing a fraction of the price of something being purchased, made to secure the right to continue making payments towards that purchase.
However, the use of "cash" is interesting. "Cash" here doesn't necessarily mean literally cash, as in Bill and Hillary dragging in large bags of money. Cash here means liquid, fully funded assets, as opposed to credit or in kind. So a check, money order, or CD would all count as cash, whereas a secured loan, or a piece of property at a particular valuation, would not.
The point is that the Clintons had access to considerably amounts of liquid assets to buy the house, which belies the suggestion that they were "broke."