An abstract is a concise summary of the study. Your abstract should contain at least your research topic, research questions, participants, methods, results, data analysis, and conclusions. You may also include possible implications of your research and future work you see connected with your findings. Your abstract should be a single paragraph double-spaced with no indention. Your abstract should be between 150 and 250 words (may vary with journal).
I would argue that describing the study is a better instruction rather than "explain" as explainations often lengthen the writing when the goal is brevity.
Here is an example of an abstract.
This study sought to examine the religion-forgiveness hypothesis in the context of the forgiveness process in an attempt to better understand the basis of the relationship. Data was collected from 140 participants in an online survey that contained measures of religious behaviors, religious beliefs, religious fundamentalism, empathy, emotional intelligence, forgiveness, and social desirability. A hypothesized model specified that religious variables of religious belief, behavioral religiosity, religious fundamentalism would be predictive of forgiveness when mediated by emotional intelligence and fully mediated by empathy. Using structural equation modeling, it was found that religiosity was predictive of greater levels forgiveness when mediated by empathy; however, religious fundamentalism was found predict lower empathy and lower levels of forgiveness. Examination of alternative models indicated that religiosity did not predict forgiveness unless mediated by empathy and although social desirability was predictive of empathy, its influence on the religiosity-empathy relationship was minimal. Implications and limitations are discussed.
The green shows the research question and an explaination of why the question is being asked. The teal outlines the methods and variables of interest in the study. Orange details the hypothesis. Yellow provides the results/ findings in addition to the primary means of analysis (Not often important, but was relevant in this application given the hypothesis). Violet indicates that implications have been considered; altough if your research is highly theoretical, you might need to have some details.