An addendum to a real estate contract or purchase agreement is a document attached to and made a part of the original contract. It can be explanatory, informational or indicate other requirements of the parties that aren't clearly spelled out in the contract.
The key is that it is executed with, and a part of, the original contract. Sometimes agents confuse addendum with an amendment, which is a modification later to the terms of an already-accepted contract.
In an area where water rights are very important, a buyer might want to make it very clear that their offer on a piece of property is contingent on the verification of legal water rights for their proposed use of the property. This could be added as an addendum to the purchase contract.