An OTP — one-time password — is a short numeric code, usually 4 to 8 digits, that a service sends by SMS to confirm you control a phone number. It's valid once and expires quickly, which is what makes it a strong second factor.
Why services send OTP codes
Verifying a phone number reduces spam and fake accounts, and adds a layer of security at login. That's why apps like Google, Telegram, WhatsApp and Tinder text you a code during sign-up.
Receiving an OTP online
If you'd rather not share your personal number, a temporary number can receive the OTP for you. Pick a number, request the code, and read it on the number's page — the code is extracted and shown clearly.