What does selling "subject to completion" mean?

I would certainly recommend that you get some clarity from the specific listing agent or seller so you know exactly what they mean by this, though it sounds like they are selling under construction and will be finishing construction prior to closing or asking the offeror to include this language in the offer.

Selling "subject to completion" means selling a property with the condition that certain tasks or improvements will be finished before the sale is finalized. Sometimes this is used for new construction or remodeling. House is put on market prior to all work being done. Sometimes this allows the Buyer to choose their own colors and fixtures to have a little opportunity for personalization.

This typically means that the home is under construction and not yet completed. It can also mean it is undergoing a remodel or some type of improvement that is not yet completed, but will be completed prior to closing.