What is Bounce rate?
The share of sent emails rejected by the receiving server; above about 2% it starts damaging your sending domain's reputation.
A bounce is a delivery failure reported by the receiving server. Hard bounces are permanent — the address doesn't exist (SMTP 5.1.x, "no such user") — and the address should never be sent to again. Soft bounces are temporary — a full mailbox, a greylisting delay, or a reputation-based rejection — and can be retried.
Bounce rate is the fastest way to lose a domain: a provider that sees you sending to addresses that don't exist concludes you bought a list. Keep it under 2%; verify addresses before sending; ramp new lists in slowly so a bad list reveals itself early.