The equation y=5 represents something we know as a constant function. In a constant function, y, always, no matter what, takes the same value and is the value of the constant. In this case, the constant is 5, it means that for all values of x, y will always be 5.
In a graph, a constant function is an horizontal line, parallel to the x axis, that cuts the y axis at y=c, c is the constant. It means that the graph of y=5 will be an horizontal line that has a y intercept of 5.