We know that the graph of a function is increasing if we get a line in a graph with a positive slope i.e. with the increasing values of x the y-value is also increasing.
The function is decreasing if the graph of a function is a line with a negative slope i.e. with the increasing values of x the y-value is decreasing.
The function is constant over a interval if the graph of the function over that interval is a straight line with zero slope ; i.e. a line which is parallel to the x-axis over the interval.