The domain of a function is the set of values of x for which a value of y exists. In this case, the only way that a value of y would not exist is for a denominator to equal to zero. If this function is f(x) = 1/(x+1) + 5, then we must find the values of x for which the denominator (x+1) = 0, which is at x = -1.
Therefore the domain is all real numbers except x = -1. In interval notation this can be written as (-infinity, -1), (-1, infinity).