Recall that the domain of a function is the set of numbers to which the function is defined. That is, if you replace the value of the independant variable (normally represented as x), then the function will give another number as a result.
In this case, we are given the function 400+13x. Since x is the independant variable, to determine the domain we must think on what possible values the variable x can take. Since in this case x represents the number of products made, it is impossible that it takes negative values, since in real life you can't produce negative amounts of products.
So far, we know that x should be greater or equal to zero. Once again, in this context, it doesn't make sense that, for example, x=7.8, since this would mean that 7.8 number of products were made. Since x represents the number of products made, it can only take values as x=0,x=1, x=2, x=3 and so on.
This set receives the name of whole numbers.