We leave the definition of an organisation to you, but it's typically a company, department or team. Every user can (and should) set-up their own organisation. Project Managers or other Admins can set-up additional organisations as required.
To set-up an organisation, select the Organisations on the left-hand-side navigation bar ...
Click the Add button on the right-hand-side ...
And fill in the pop up form ...
As you'll notice, only the Organisation name is mandatory. You can always come back and update the additional fields later.
And also note you can also change the organisation name itself later on if needed.