The domain is all the values of the independent variable (in this case, x) for which the function is defined.
In this case, as it is indicated with the arrows in both ends, the function continues for greater and smaller values of x.
As there is no indication that for some value or interval of x the function is not defined (a discontinuity, for example), then it is assumed that the function domain is all the real values.
Example function:
We have the function y=1/(x-2)
We can look if there is some value of x that makes the function not defined.
The only value of x where f(x) is not defined is x=2. When x approximates to 2, the value of the function gets bigger or smaller whether we are approaching from the right or from the left.
Then, the function is not defined for x=2. So, the domain of f(x) is all the real numbers different from x=2.
The domain is, by default, all the real numbers, but we have to exclude all the values of x (or intervals, in some cases like the square roots) for which f(x) is not defined.