It means that the property that you are looking at is either sold by a builder/developer or a bank. Builder contracts usually always have an addendum, or additional page/ pages that are added to become a part of the binding contract. The terms covered in these addendums could be anything, but usually are clauses that benifit and protect the builder/ seller. The same can be said for bank owned property. Their addendums are very strict and cannot be altered, and are meant to indemnify them, and protect their interests.
You must agree to these addendums if you want the property, at least with a Bank Owned Property. Sometimes a builder/ developer/ seller will allow changes to the addendum, but typically if you do not want to sign the addendum, then you need to find another home to buy. You should read these addendums to know what you are signing. Have your Realtor explain it to you if you don't understand the meaning of the addendum.
All the Best, Dan