An OTP is a short numeric code sent by SMS that is valid once and for a few minutes, used to verify a phone number or approve a login or a transaction.
One-time passwords are the most common transactional SMS. The code is generated by your system, sent to the number the user gave, and checked when typed back. Its value is that it proves possession of the phone at that moment, which is why it expires quickly and is invalidated when a new one is issued.
What makes OTP traffic succeed is speed and format: brand first, code second, one segment, no link, short validity, and a resend policy that does not let a bot drain your balance. Where a country forces a numeric sender, the brand name in the text is what tells the user the code is legitimate.
At SMSMeteor: send codes with validity set to the code's lifetime and the transactional flag; the OTP guide covers rate limiting and fallbacks.