It sets out the purpose and the rules of your club. This is the document that helps to make sure that things are run properly. In general, this documents sets out:

• the objective(s) of your club – what you want to do or provide;
• the different types of membership – adults, juniors, social;
• the rules under which your club operates;
• how the affairs of the club will be managed;
• how the members control the club – normally through an Annual General Meeting (AGM).

A template of a constitution can be found here -  but this is only a template as not all sections within it will be relevant to every club.