A downpayment is an upfront investment made by a borrower, and in most cases it is required.
A common downpayment for purchasing a property is 20% of the total cost of the property, this is often done to keep the Loan to Value at or below 80%.
Not all types of loans require a downpayment, however. The most well known case of this is for a Veterans Affairs (VA) Loan.
VA Loans are loans for military members and veterans and do not require a downpayment, but they have their own restrictions.