It means that if you are in jail and think there might be other charges pending against you, you should notify the District Attorney that you are demanding to be brought to trial on those charges within 90 days. The purpose is to avoid the situation where you serve your entire jail sentence on one charge, and the day before you are due to be released, the DA starts prosecution on another charge that was pending. If you make the section 1381 demand, you can at least serve some or all of the sentence on the second case concurrently with the sentence on the first case.