A Cutie Mark is meant to represent the personality and special talent of each pony that has one. Cheerilee's flowers symbolize her desire to help her students bloom, and the smiles represent the cheer that she hopes to bring to their lives.

At least...that's what she tells everypony who asks.

[SEMI-SAD]

Star Dreamer and Twilight Sparkle never used to be kind of ponies to have friends, but they've always had each other. As adults, they find that their friendship is the most important thing to happen to either of them.