First argument:

AllRats | Create rat lines for all loaded nets that aren't already connected on with copper.
SelectedRats | Similarly, but only add rat lines for nets connected to selected pins and pads.
Close | Selects the shortest unselected rat on the board.

The second argument is temporary and optional, for a workaround of a bug in the old (to be removed) autorouter. If the value of the second argument is "manhattan", non-axis-aligned lines are ignored when looking for rat anchor points.