The resource world (or 'resource') is a world that exists solely for harvesting resources. It exists to discourage people from making the main world less aesthetically pleasing, by giving them an area in which they can mine, dig and chop to their hearts content. Every month the world is reset, so it is discouraged to build permanent structures here.
You can get to the resource world by using the /warp command ('/warp resource') or by using the /rtp command ('/rtp resource'). The second method is recommended as it will teleport you to a random and, usually, less used area.