The REST API is a mechanism for accessing online services using HTTP requests to obtain, change, and remove data, depending on the REST architectural style. API security is crucial because it protects sensitive data and business logic exposed to third-party apps and services through APIs. Common API security threats include man-in-the-middle attacks, code injections, and DDoS assaults. To secure a REST API, developers can use techniques like Basic-Authentication, DIGEST Authentication, Client CERT Authentication, OAuth2, and implement best practices such as using API Gateways, authentication, Transport Layer Services (TLS), frequent security testing, validating API data, updating API services, and using Web Application Firewalls (WAFs). Security should be considered throughout the API development lifecycle to ensure that APIs are both functional and secure.