c is the Y-intercept, or where the line crosses the Y axis.

So if you have the equation y = 1x (ie. m = 1) then you have a line that starts at 0 and for every one unit that you go across the line also goes up one unit. If you have the equation y = 1x + 5, then your line looks exactly the same but has been moved five units up (easy to see if you draw it or just plug in some x and y values)