A seed list is a set of real or monitored mailbox addresses spread across major providers (Gmail, Outlook, Yahoo, and others), used to check inbox versus spam-folder placement, image and link rendering, and authentication pass/fail for a specific message.
Seed testing measures placement for that one test send at that moment; it is a diagnostic sample, not a guarantee about how the same campaign lands for an entire, larger recipient list with different engagement history.
It's most useful for catching authentication regressions, content-filtering triggers, and rendering breakage before a large send, and as an early warning signal when inbox placement for a domain has quietly degraded.