Getmap is a CubeScript command that retrieves the map (as long as it isn't secured) currently being played for the local client. Since AC version 126.96.36.199 the client is required to have the exact same map as the server to play in non-coop modes. This was result of a very easy way of cheating by just playing on a modified version of the map.
Some issues stem from Mac users using the packaged version of 188.8.131.52 as a couple of the official maps are of a different revision than the rest of the clients. Being official and secured maps, Mac users aren't able to play online if the server is using this map. These two maps are ac_arabian and ac_toxic. In order to download the correct versions of the files, visit this page.
Related CubeScript commands are autogetmap and autogetnewmaprevisions. autogetmap removes the need to manually type /getmap when you enter a server requiring you to download a map, and autogetnewmaprevisions removes the need to /getmap when you enter a server with a different revision of the map you have. sendmap A sends map "A" to the server so it can be played.
Direct download links to affected maps: