You can conquer anywhere you want to.
These maps show the tiles that if your civ owns will penalise you the least for your expansion stability (because I understand that all tiles after a certain (small?) number attract a slight penalty and some ahistorical tiles just attract much more of a penalty than others).
The maps are also are related to which areas the AI will try to colonise if they were playing that civ, however I think that there is also code which encourages the AI to found specific cities during the game - such as the Euros to found a colony where New York will eventually be and the French to found Fort Detroit and New Orleans.
Actually, they specifically are the priorities for the AI to send settlers to, and they do not tell you specifically which tiles to own to be historical. Note for instance, that some maps don't allow for natural cultural expansion (e.g., Carthage). However, generally speaking, if you settle along the map lines, you'll expansion stability will be better. Note the subtle difference between what this and what blzzrd said.
Furthermore, this doesn't adequately describe AI war preferences, or historical war expansion. Note that the German map doesn't include France, but I can't imagine France not being part of the German historical map.
I don't know what the light green means.
I think the colors represent different settling priorities. I'm not really sure what the different colors specifically mean, but I would guess that yellow > green > red > purple > dark gray > light gray > white. Green looks like maybe a first-ish priority for colonial expansion, but I don't really know. So you see Portugal trying for Rio de Janeiro first and England going for Newfoundland.
Bottom line: they give you a very rough idea what historical expansion is, and a pretty good idea about colonial expansion. They do not tell you specifically where to expand to be historical, and only use them knowing that.