Historically, bonds were printed on paper with detachable coupons. The coupons were presented to the bond issuer by the bond holder to collect periodic interest payments. The Excel COUPNUM function returns the number of coupons (interest payments) payable between the settlement date and maturity date.
The settlement date is the date the investor takes possession of a security. The maturity date is the date when the investment ends and the principle plus accrued interest is returned to the investor. The frequency is the number of interest payments per year. Basis specifies the method used to count days (see below). In the example show, the formula in F6 is:
In Excel, dates are serial numbers. Generally, the best way to enter valid dates is to use cell references, as shown in the example. To enter valid dates directly, you can use the DATE function. Below is the formula in F6 reworked with hardcoded values and the DATE function:
With these inputs, COUPNUM returns the same result.
The basis argument controls how days are counted. The COUPNUM function allows 5 options (0-4) and defaults to zero, which specifies US 30/360 basis. This article on wikipedia provides a detailed explanation of available conventions.