Any time evidence comes in, it has to be authenticated by a witness. That means someone has to get on the stand and testify that it is what it purports to be. One manner of getting around that is to have both parties to the case stipulate to their authenticity. In doing so, Jason has to tell the Judge that he understands what he's giving up by agreeing to stipulate to the authenticity. He is allowing the Prosecution to introduce evidence without requiring them to go through that extra step. It's a good tactic, because it saves time for both parties, and it makes things flow a lot smoother. The Judge requires JY to testify that he understands the legal ramifications, so that it is not an appeal-able issue.
It doesn't mean both sides agree on the relevance of the evidence, just that it is what it purports to be. Usually done for records (like the HT records in the Cooper case) or bills, etc. Because otherwise, you'd have to get someone in from American Express to testify to how those records were collected, etc.
