A bearer token is a credential sent in the HTTP Authorization header as \"Bearer <token>\"; whoever holds it can use it, which is why it must travel only over HTTPS and be stored as a secret.
The bearer scheme is the simplest HTTP authentication: the client presents the token, the server checks it, no signature of the request is involved. It is convenient and universal, and its security rests entirely on transport encryption and secret handling.
Compared with request signing, bearer tokens are easier to integrate and just as safe when the token is scoped, rotated and restricted by IP.
At SMSMeteor: every request carries Authorization: Bearer sk_live_…; the API is HTTPS only and rejects unauthenticated or wrongly scoped requests with 401 unauthorized.