(x^2-25)/(x+5)=(x-5)(x+5)/(x+5). The factor (x+5) cancels and we're left with x-5. But we can't remove the common factor if x=-5, because we'd be dividing by zero which gives us 0/0 and is said to be undefinable. But all other values of x are allowed, which means as x gets very close to -5 the result is still x-5, which means we get a result close to -10 every time. So we can say that, although the expression isn't really definable at exactly x=-5, the result is -10 at this point just like it's exactly x-5 everywhere else. If x<0 the result x-5 has a maximum value of <-5 (that is, not quite -5) and no definable minimum value, so we call it minus infinity (or "minus indefinitely").