Sender replacement is the substitution, by the route or the operator, of the sender you specified with a number or a shared sender, because the destination does not deliver the original.
Replacement happens for three reasons: the country does not support alphanumeric senders, the name is not registered where registration is required, or an operator rejects a specific name. The message still arrives; the recipient sees a number or a generic sender instead of your name.
Honest reporting shows which sender was actually used per message. Silent replacement is one of the ways a cheap route hides its behaviour.
At SMSMeteor: the expected behaviour is stated per country before you send, and the report's from field reflects the sender actually used after any replacement.