It highlights your achievements by drawing together different examples of your expertise and accomplishments from various employment and educational experiences. It focusses on the Experience section, where each job is listed in reverse chronological order and described in some detail. Think broadly about your experience - for example, working for Big Bad Burgers is not chemistry, but it may demonstrate other attributes such as reliability and leadership.
First impressions are important.
It should contain absolutely no errors. Make sure you've checked with your referees before putting their names down. Recent professional referees are normally best. This is irrelevant to professional employment in chemistry. The summary consists of several concise statements that focus the reader's attention on the most important qualities, achievements and abilities you have to offer, and why they should hire you instead of the other candidates.