The domain is the kind of numbers that can be put into the function.
The range is the kind of numbers that result from solving the function.
f(x) = x² + 3.
Any real number, positive or negative can be substituted into the function.
But when the function is carried, out because of the x² , the result would come out as positive number greater than zero.
Therefore the domain of f(x) is any Real number, R
The range, is a set of Real numbers greater than zero. R > 0
Hope this helps.