The Foundation Rule
The rules of evidence limit what a judge can consider at a hearing. One of those rules says that a document must have a "foundation" before the judge can consider it.
What is a Foundation?
Foundation is other evidence that shows that the document is what the party offering it to the judge says that it is. In most cases, this is pretty easy. Before you ask the judge to consider a document, you need to explain what it is and where you got it.
For your checking account statement, for example, you should tell the judge that this document is a statement from your checking account, what bank it is from, that you received it in the mail (or printed it after logging into the bank website) and that it is a true statement of your checking account based on your own knowledge. That's a foundation.
What do I do if the other party tries to offer a document without a foundation?
You need to object to this evidence. Stand up quickly and say "Objection, lack of foundation." Usually, the judge will then ask a couple of questions to help the other person describe where the document came from.
TIP: Often, it is not worth objecting to evidence based on foundation. The judge will just ask a couple of questions to help with that problem. Save this objection for any important documents that you believe are not actually what the other party is pretending.