In order to write a summary of a poem, one must establish the meaning of the poem. To do this, the reader needs to find the controlling metaphor. That is, the tension between the literal and the figurative=the main unstated comparison. This controlling metaphor contains the moral lesson of the poem.
For instance, if one were summarizing John Keats's "Ode to a Grecian Urn," he/she can perceive that the tension exists between the speaker's appreciation for the eternally preserved beauty of the lovers on the urn and the sense of sorrow that his heart feels at looking at it because he realizes that the urn is not life. The lovers, while beautiful, cannot engage in the act of fertility that continues life. And, so, the lovers on the urn are compared in the mind of the speaker to real-life lovers, and there lies the tension/controlling metaphor.
Likewise, the reader of a story looks for what is unstated behind the narrative. This is the theme, the moral truth, that the author wishes to convey. Often a reader understands intuitively this truth, but putting it into words is somewhat difficult. For this reason, it is helpful to many readers to record their first impressions immediately after they finish reading. Then, they can read these and reflect upon the narrative again. Often a second reading is the solution for finding the theme. Since the reader is familiar with the plot already, he/she can put attention solely on the element of theme with the second reading and will find important points missed on the first reading.
Now, a critical summary is much like a review: you give a summary with some critiques on salient points. Usually one of the elements helps to establish the theme. Perhaps, the point of view is effective for the theme, or the setting or one of the characters is pivotal to the theme, etc.
Please see the enotes site below for instruction on how to write a summary in 8 easy steps.