Zoomable values allow you to specify how the certain property will change its value according to the zoom level. Basically, you specify the property values for certain zoom levels, and the rest is calculated (interpolated) by Maperitive based on known points you specified.
line-width : 5:1;9:1.5;17:6
means that the line width will be 1 pixel on zoom level 5, 1.5 pixels on zoom level 9 and 6 pixels on zoom level 17. Everything in between is interpolated, so for example on zoom level 12 the width will be 3.1875. For zoom levels below or above the specified points, the first or last specified value is used, so in our example
You can also specify a constant value:
line-width : 5
which is equivalent to
line-width : 1:5
line-width : 10:5