A process server gave him a summons to appear in court to give testimony. Once done, the person served no longer has a an excuse to not attend court. If he doesn't, the judge may take more active measures.
When a judge summons you to court to give testimony, it's not a request. Some of the popular lame explanations for ignoring a judge is to say that you didn't get the letter, or that you didn't receive the summons for whatever reason. Much of the time, it's an evasion tactic to not appear before a judge. The workaround is that you get served. A process server will find you, identify that it is in fact you, and then he will hand you the summons. He will usually say "You have been served". In order to make sure that the summons cannot be contested, a witness is involved (I do not think that is a requirement however).
If the person being summoned is aware that a judge is looking for him to give testimony, he may make himself hard to find. He may identify himself the wrong way on purpose. He may refuse to put out his hand for any reason. Process servers may use various tactics to entice these people to identify themselves and accept what is handed to them. Once done, the process server usually leaves as there is no reason to stay and no reason to invite an angry retaliation.