Each breed has a standard of perfection. This details what an ideal rabbit of that breed should look and feel like. Show quality rabbits are the ones that fit this standard as closely as possible and should do well at shows (no guarantees as it does depend on the other rabbits show on that day and the judge).
Each breed also have faults. These are things that are not wanted, but can still be ok in a show. It it usually markings, but can be other things.
There are also disqualifications or DQ's. These are things that will get a rabbit kicked off the table (not literally, but they won't be allowed to compete). These can be markings or lack of marking, colour, toe nails being the wrong colour, tooth problems or other issues. DQ's will make a rabbit unshowable, but some can still be good for breeding depending on what it is and if it can be passed to offspring.
If you are looking for a show rabbit, it is a good idea to get an idea of what the standard for that breed it. Also know faults and DQ's as they are important as well. Then find a good breeder who will go over the good and bad points of a rabbit with you.