There are several examples of this from the usual sources, ShopNotes, Woodsmith and Popular Woodworking plus the jigs and fixtures collections that are available. The premise is that a sub plate elevates the router allowing the bottom of the plate to rest at the desired level and the bit to pass over a proud surface, like edge trim, to bring it flush.
I have an old battle tested offset base that, as you can see from all the holes in it, has served more than its share of purposes. All the extra holes make this a little hard to look at but, here goes;
Here is a router mounted to the offset base. Most versions of this jig call for you to make this offset base as part of the effort; I just happen to have one that is "well used" and I don't mind a few more holes:
I cut a piece of acrylic to the shape I am after. The important part is the area approaching the bit. It should angle away to each side to allow pivoting the motor for stability and support during the cutting operation. This angle-away profile is also what allows you to sneak up on corner areas of wrap-around trim. The rest of the sub base could be any shape that won't get in your way. I double taped my blank to the original offset base and flush trimmed it on the RT.
I then drilled through holes in the offset base and tapped receiving holes in the sub base:
It goes together something like so:
Set the jig on the surface you wish to level to. In this example my workbench surface represents a table top adn the piece clamped in my vise represents some trim I may have glued around it.
Here's a close up of the bit/sub base relationship:
So the reference surface is on the left in this pic, the proud material held in the vise is brought flush with the reference surface. In the real world, the trim would be glued to the reference surface so, ignore the vise jaw:
The general idea of this jig is to allow flush trimming where you would not want to hold the router perpendicular to the edge (awkward) or you fear damaging the reference surface's veneer.