Samech stands for "Somech Noflim," "[Hashem] supports the fallen." Its shape is a circle, and stands for the different highs and lows of life. This is very much the theme of Purim. Not only did Hashem support us when we had fallen, but also the story itself comes "full circle," with Mordechai and the Jews (who were down) brought up, and Haman (who was up) brought down.
Ayin means "eye," as well as "well," "fountain," "spring." The eye is known as the "window to the soul," shedding light on a person's inner dimension. Similarly, a wellspring represents the revelation of the hidden, inner spiritual aspects of the earth, its deeper waters. (See Sixth Set of 22-days) This is also one of the themes of the month of Adar, revealing (Megilah) that which is hidden (Esther).
Both above themes, a) relying only on G-d for support, whether in a high or a low, and b) revealing the hidden spirituality with everything, can be found in the song of the Stars:
The Stars are saying, "You, only You, are God; You made heaven, the heaven of heavens, with all their hosts; the earth, and everything that is in it; the seas, and everything that is in them; and You preserve them all; and the host of heaven prostrate themselves to You." (Nehemiah 9:6)
Stars themselves also offer us a glimpse into the vastness of the universe. They grant us a better sense of our smallness and of G-d's greatness. The name Esther comes from the word for star. She is referred to in Psalm 22 as Ayelet HaShachar, the "Morning Star." The morning star is the very last one to shine before daybreak. Similarly, Esther is the last prophet(ess) before the coming of Mashiach and the final redemption.