When you or your spouse files for divorce, one of you will receive the divorce petition. You or your spouse will be served the documents and be required to respond to them. You'll have to respond within around 30 days or you lose the right to argue for your preferences to do with child custody or property division.
In the answer to the divorce petition, you'll be acknowledging that you've received the document. You may find statements and information inside that you need to review. The forms may include information about your marriage and spouse along with certain requests, like a request for custody or alimony.
You'll be able to agree or disagree with the documents in the divorce petition. At this point, most people choose to work with an attorney, so they protect their interests. However, there are some pre-printed court forms that make it easy to respond "admitted" or "denied" to the requests. You can also write your own demands on those forms.
If you do not answer a divorce petition, you are willingly giving up your right to make decisions. The court will assume that you agree to anything your spouse put in the documents and then enter a "default" in the case. That means that you no longer have a right to argue against your spouse's claims.
It's not a good idea to allow a default. Take the time to look over the documents, but return them with your information in a timely manner. You want to retain your right to argue and negotiate, not give your spouse anything he or she asks for.
Source: FindLaw, "Answering the Divorce/Dissolution Petition," accessed Feb. 16, 2018