1. I know it’s short notice but you’re welcome to join us for dinner if you don’t mind taking potluck.
2. There isn’t much choice left, but I’m not picky. I’m happy to take potluck.

The expression probably originates from the idea of a potluck dinner, where each guest brings one different food or dish to contribute to everyone’s meal. Nobody knows in advance exactly what foods will be brought. Potluck dinners and potluck picnics are popular in the U. S. They are informal gatherings and easy to organize because no one person has to provide more than one dish.