Material Overrides allow you to edit materials inside complex/deep mesh structures like SVGs or Text. This can be achieved via the Sub-Mesh Behaviour's Fill or Stroke tabs.
The Swap Color Override enables you to take one specific color within a Sub-Mesh (e.g. an SVG, Text Shape or Duplicator) and swap it for another one.
Old Color - set the color you want to swap.
New Color - set the color you want to swap to Old Color to.