A loose jar lid, a night full of fireflies: the poem opens on a small, quiet moment where something has slipped free. The lid isn't described as being opened on purpose, so the release feels gentle, almost accidental. Then the fireflies rise and, the speaker says, shine brighter than the stars.
The most interesting line is the one about "memories once caught." It suggests the jar held more than insects. Judging by that line, the speaker has been keeping memories shut away, the way a child keeps fireflies, and now they glow again. The repeated "that night" makes the evening feel precious and fixed in time, as if the speaker can still point to it exactly.
The mood is wonder, with a touch of wistfulness. The exclamation marks show real delight, but the idea of letting go, or of memories escaping, carries a soft sadness too. It's a short, bright snapshot of nostalgia.