Well is one of the two what I like to call “tug of war” style of maps that shipped with Team Fortress 2, along with Granary. Basically the maps contains 5 linear control points, and each team starts controlling 2 with the middle one beginning neutral. Both teams fight of the middle point, and once it is captured, only 2 control points are in play at any one time, both teams’ furthest forward one. The ultimate goal is to capture all 5 control points in order.
A couple of things worth noting that mix up the achievement of this objective however is firstly that each control point has a different capture time – that is the middle point takes the longest to capture, whereas a single player can capture the final points in around 5 seconds. Also holding more than your two initial points (i.e. controlling at least the middle, maybe some enemy points) grants your team a decreased respawn timer and allows you to spawn further forward, reducing transit times to the action. See my article on map control for a look at the importance of this.