The poem uses several metaphors to convey the mood and experience of its characters, particularly the "Young mothers" who don't feel so young anymore.
First, in line 1, the speaker announces that "Summer is fading." This is a bit more figurative than a phrase like "Summer is ending"; seasons don't literally fade away. This choice of words subtly evokes other kinds of fading, both literal and metaphorical: for example, the fading of youth and "beauty" (see line 22), or the fading of analog films and photos with the passage of time. In short, the word "fading" helps establish a wistful, nostalgic atmosphere.
The same stanza describes young moms and their kids gathering at the playground "In the hollows of afternoons." This metaphor suggests that they head outside during gaps ("hollows" or empty spaces) in their afternoon schedules. But the word "hollows" might also have an emotional layer, suggesting that these mothers feel empty in the afternoons—as if something's missing from their lives.
Indeed, the poem ends up exploring their sense of wistful unease. The poem's final metaphor captures exactly that feeling:
Something is pushing them
To the side of their own lives.
That is, they've sacrificed so much for their families that they no longer feel like the central characters in their own stories. They may also feel "push[ed] [...] To the side" by time and nature, as younger generations—including the "lovers [...] in school" and their own small "children"—grow up and take over the world. But, of course, nothing's literally pushing them; the sidelining is entirely figurative.