The colon operator may be used as a short-cut when specifying a path in the DM code tree. Instead of specifying the full path, you can insert a colon and the compiler will search down in the tree with the node you specify. This is known as a “downward” search. You should only use it when the target node is unique.
The following example demonstrates the principle but it obviously doesn’t save much typing!
world
mob = :player //short-cut to /mob/player
mob/player
Login()
src << "Welcome, [name]."