When you receive a divorce or dissolution petition, you must answer it. The petition itself is first filed with the court, and then the divorce petition is served to the other party. Being served simply means you have been legally delivered the paperwork. As the respondent of the paperwork or defendant in the divorce process, you must respond to the petition within the selected time frame. Most of the time, it's 21 days.
Once you respond, you also need to discuss the allegations in the paperwork and make your arguments. If you don't do that within 30 days of receiving the paperwork, you won't be able to argue for what you want or need in issues of child custody or property division -- two areas you likely will want to have a say.