Within MekWars, the map can refer to two separate things: the campaign map within the MekWars interface, and the game map within MegaMek.
MekWars Campaign Map
The MekWars campaign map, found on the "map" tab of the MekWars interface, shows the current borders and territory of each faction. By clicking the various stars/planets, additional information is presented, including planetary hold percentage by faction, possible terrain chances when battling on a given planet and other planet-specific information, like whether it's a capital world, has a factory, etc.
By clicking a planet, the view will center on that planet which gives you the planet name, faction holder and a few more pieces of information in the top left. If enabled, circles representing attack range (based on Operation) will be shown from the selected planet. By double-clicking the planet, a pop-up window will further detail exact faction influence/ownership, Production / Industrial statistics, planetary conditions and Planetary Terrain. Each terrain will have a given percentage which represents the chance of a map being generated using those conditions.
Use the campaign map interface to declare attacks when a target presents itself and to coordinate as players work towards their faction's goals.
MegaMek Battle Map
The map within MegaMek is the specific map used for a battle between two or more opponents. This hex map is what Classic BattleTech is played on and each hex has a specific Terrain which affects how units move through, see through and behave in.
MegaMek also features a handy minimap which helps with big-picture analysis of a battle.