According to Senior Network Engineer, Michael McNamee (SecurEdge Networks), roaming is, "the process by which a client device or wireless device moves and shifts its connection from one access point to another as it moves throughout a facility or across a campus."
So for instance, let's say you have 3 access points around your warehouse facility. As you move about the coverage area with your wireless device, there is a hand-off from AP 1 to 2 to 3- seamlessly.
In a well-designed network environment, you should experience seamless connectivity as your roam- maintaining voice or video session etc. without drops in wifi performance (i.e. choppy video, frozen video etc.).