Swamps is a terrain type that is walkable by creeps. Though it is walkable terrain, it has an increased movement cost of 2 compared by plain lands. This makes swamps less interested for creeps who intend to use the shortest path.
Swamps pose interesting tactical and strategic challenges for players. The increased movement cost makes roads extremely useful for cross room hauling and remote mining.
Militarily, swamps provide players with a dilemma- creeps are unable to maneuver properly when fighting in swamplands. This can provide some players with strong defensive positions, for example when attacking creeps must path around or through swamps, breaking up their formation or exposing them to enemy attack for a longer period of time. (For an example of this in action, look here).
Be sure to consider the terrain of a room before spawning there. While at higher RCL levels the movement cost can be heavily mitigated through the use of Links and Roads, the process of getting to that point can be difficult for new players and those without the support of an already-developed room.