It just means that the lengths of the sides of the triangle are x, 3x, 8x, where x is any positive number. The given ratio is supposed to create a triangle of a particular shape and all triangles with side lengths x, 3x, 8x would have the same shape, so all would be similar triangles.
However, this triangle cannot exist because it breaks the Triangle Rule that the sum of the lengths of any two sides is greater than the length of the third side. The rule must apply to all sides.
x+3x=4x which is not greater than 8x; 3x+8x=11x which is greater than x; 8x+x=9x which is greater than 3x. So the Triangle Rule doesn't apply to all the sides, therefore the ratios do not produce a triangle.