The objective or purpose in an abstract states (as simply as possible) the motivation for the project. It answers the question: why did I undertake this project? Although you don't necessarily want to include all of your background information, explain why current scientific information does not answer the problem you are trying to solve. In other words, detail the necessity of your study.
After the purpose, include an explanation of the methods of your study. Stay broad; although you detail specific methods in your paper, only include generalities in your abstract.
For example, if you tested for pupil dilation according to iris color using a high-tech camera, explain that you used a camera and how you used the information from the camera to achieve your results. Save the type of camera and your methods to prevent data corruption for your actual paper.
Outline the results of your experiment in your abstract, after your methods. To do this, give a general statement of your findings, rather than the exact number of the outcome.
For example, do not say: "Photosynthesis increased by 202.3 percent in oak trees after a forest fire." Instead, state: "We found that forest fires had a positive impact on the photosynthesis of certain trees."
Lastly, an abstract allows you to draw conclusions based upon your results. While your experiment and accompanying paper state the "how" of a project, use the abstract to explain the "why" of the conclusions.
If you found that photosynthesis increased in plants after forest fires, state (backed by research, of course) your belief that the increase in available light to surviving plants increased photosynthesis.