A concept note is the shortest expression of your project idea on a paper. It is a brief written note, giving an introduction to your project and your organization. If your NGO is seeking to raise funds for a project, then the first thing you would do is develop a short note of this project, explaining what it is and why it is required.
Most donor agencies interested in providing funding support to NGOs prefer to receive an outline from them about the prospective project. As proposals are lengthy documents and competition is high, donors now do not have the time to go through a large number of applications. Therefore, they ask NGOs to initially submit the project outline in form of a concept note.
Concept notes usually are 2-3 pages in length and comprise of the situational analysis, project rationale, objectives and results. A broad indication of the budget required for the project is also mentioned in the concept note. A profile about the organization is also useful.
Some donor agencies can have specific formats for a concept note and there can be lengthy requirements. However, in most case, the above sections are enough to give an overview of the project.