Key Flat Rate in refers to the flat r associated with this curve.
Enter .04 for a 4 % rate.
Here the rate r assumes the daycount and compounding conventions defined in the entries and respectively.
Concretely, the curve is built in such a way that it implies the same zero rate r for any maturity, provided the daycount and compounding conventions are as above.
Note that setting a flat zero rate here does not mean that the implied zero rate with respect to differing daycount or compounding conventions is also flat!
Also note that the implied forward rate - even with respect to the same daycount or compounding conventions - will generally not be flat either.
An exception is when the compounding convention is set to continuous, in which case the continuous forward rate will also be flat.