No TL;DR found
A Civil Society Organisation (henceforward CSO) is formed when a group of people decide that they would like to associate around something which they care about. In some cases their decision is made for them as, for instance, the members of an age group in many African countries, or a person being born a Jew or a Muslim, but in many cases, probably most, a CSO is formed from people who voluntarily decide to associate together for some purpose which is important to all of them. In the jargon, this is their “Mission”: it may be to listen to music, it may be to play football, it may be to join a cooperative, it may be to support a charity, it may be to lobby for changes in the law.