It acts similiarly to Excel's IFNA function. If value is NA(or blank in Excel terms) then the function will return the second argument, if not then it will return the non-NA value which is the first argument.
Usage
IFNA(value, value_if_na)
Arguments
value
Evaluate if it is NA.
value_if_na
What should the function do if the value is NA.
Value
In this case the function returns "It is NA" as we specify the first value NA. Function can return different classes because first argument can be either character,numeric, logical or anything else.