Channel surfing (Channel Changing) is to move from channel to channel for a purpose. For example, one would channel surf to find an empty map.
However, when in a map with one or more other players, they may request that one "cc plz"(Change channel please). It could be the reason that they want to train undisturbed. These requests are especially common at maps where bosses spawn. It is advised but not required to follow the requests to avoid any negative relations between you and that player(those players).