A governing document (or constitution) is the document that sets up an organisation and says what its purposes are. It will usually deal with other matters, including who will manage and control the organisation, what their powers are, what they can do with the organisation's money and other assets, and membership of the organisation.
What your organisation's governing document is called and how it is put together will depend on what kind of organisation it is (what its legal form is). If it is a SCIO it will be called a constitution and must contain certain clauses. If it is a company the governing document is a memorandum and articles of association. If it is a trust governing document it will be a trust deed or similar document. If your organisation is an unincorporated association it may simply be called your constitution.
The Scottish Council for Voluntary Organisations (SCVO) has model governing documents for the different legal forms which will usually be acceptable to OSCR. There are also model governing documents for specific types of charities, like Playgroups or Housing Associations. If there is already a model available we recommend that you use that.