Sender ID registration is the process, required in some countries, of declaring an alphanumeric sender name to the local operators or regulator before it is delivered as typed.
Countries such as India, the Gulf states, Turkey, Indonesia, the Philippines, Nigeria, Kenya and others require names to be registered, sometimes with company documents and sample messages, sometimes only through the gateway. Lead times range from days to weeks. Until approval, traffic goes out from a shared numeric sender, so nothing is blocked; only the branding waits.
Registration is per name and per country, occasionally per operator. Some destinations do not accept registrations from foreign senders at all.
At SMSMeteor: support prepares the registration file with you where the destination accepts foreign senders; the coverage page marks these countries "registration required".