Builders finish is a basic finish. Sanitary ware included, no painting, no tiling, basic wooden flooring and basic internal doors etc. If you are intending on buying it check these things if it is newly built. Make sure the insulation is up to scratch and that windows have the argon filled gas in between panes - these are fairly standard things nowadays to ask for. Also don't forget to ask for the ber rating of the house again. This is standard practice if it is a newly built house. With regards to finishing the house add on 30% to the price you are paying to give a rough idea of what it costs to finish the house.
If you are referring to material used to build or improve a home, builders grade is the very basic in quality and finish. The material may look good when new but will not hold up as long as a higher quality and more costly material. These items include, windows, flooring, cabinets, vanities, lighting, plumbing fixtures, doors, etc. As a builder of custom homes (retired), I would not even consider using builders grade items in my customers homes.