Since topographic contours represent points of equal elevation the contour elevation tells you the elevation difference between two adjacent contours. Generally contour intervals are in units that allow you to see the elevation differences. Larger contour intervals are typically used in regions with large differences in elevation (e.g. mountainous regions) conversely regions of very small topographic relief (e.g. coastal plains) will have a smaller contour interval to show more detail. On USGS every 5th contour is usually shaded dark brown.