The 'm' value should be used to provide a gradient to the graph (i.e. for every 1 unit the graph moves along the x axis, its y movement will be altered by multiplying 1 by 'm'). Therefore, the higher the 'm' value, the steeper the graph, while the closer it is to zero, the less steep it wil be. Note that a negative 'm' value will result in a graph that slopes downwards.
The 'c' value denotes at what point the graph crosses the y axis. Like 'm,' it can be negative or positive, simply moving the whole graph up or down.