What does the function 'double' do?

Answer 1

double is not a function. It's a floating point data type which is used to declare a variable of size 8 bytes in modern compilers

Answer 2

It is not a function. Type double is a floating point type that is larger than or equal to type float, but shorter than or equal to the size of type long double.
Range of values:
1.7E +/- 308 (15 digits)