The different types of optimizations change your board in order to reduce the total trace length and via count.

Arguments:

debumpify Looks for U-shaped traces that can be shortened or eliminated.
unjaggy Looks for corners which could be flipped to eliminate one or more corners (i.e. jaggy lines become simpler).
simple Removing uneeded vias, replacing two or more trace segments in a row with a single segment. This is usually performed automatically after other optimizations.
vianudge Looks for vias where all traces leave in the same direction. Tries to move via in that direction to eliminate one of the traces (and thus a corner).
viatrim Looks for traces that go from via to via, where moving that trace to a different layer eliminates one or both vias.
orthopull Looks for chains of traces all going in one direction, with more traces orthogonal on one side than on the other. Moves the chain in that direction, causing a net reduction in trace length, possibly eliminating traces and/or corners.
splitlines Looks for lines that pass through vias, pins, or pads, and splits them into separate lines so they can be managed separately.
auto Performs the above options, repeating until no further optimizations can be made.
miter Replaces 90 degree corners with a pair of 45 degree corners, to reduce RF losses and trace length.