A concept note for a project is a short expression of the purpose of a project for which you wish to receive donations and/or approval.
It should include:
The name of your organization;
A title for the project
Context and rational for the project (why are we doing this?)
Strategy and activities that will be used to obtain the goals.
It also might be good to explain why this project is different from earlier, similar projects, if any.
There are other items, depending on the project. Look up "concept notes" in an internet search and you should find some good sites.