An executive summary is the first part of a business plan. It should be concise--no longer than a page. An executive summary is different for a new business and an established business, but it is generally used to situate your business within a market by describing your products or services, where you fit in the competitive landscape, and your future plans.
From the Small Business Administration:
If You Are an Established Business
If you are an established business, be sure to include the following information:
With the exception of the mission statement, all of the information in the executive summary should be covered in a concise fashion and kept to one page. The executive summary is the first part of your business plan many people will see, so each word should count.
If You Are a New Business
Demonstrate that you have done thorough market analysis.
Include information about a need or gap in your target market, and how your particular solutions can fill it.
Convince the reader that you can succeed in your target market, then address your future plans.