Trusted Firmware-M (TF-M) is the reference implementation of Platform Security Architecture (PSA). PSA is a recipe for building secure connected devices from analysis to implementation. PSA consists of three elements - Threat models and Security Analyses, Architecture Specifications and Open Source Reference Implementation (TF-M). See PSA Resource Page
TF-M provides a highly configurable set of software components to create a Trusted Execution Environment. This is achieved by a set of secure run time services such as Secure Storage, Cryptography, Audit Logs and Attestation. Additionally, secure boot in TF-M ensures integrity of Run time Software and supports firmware upgrade.
Functional Block diagram of Trusted Firmware-M below shows the various components and how it would fit in a v8-M System. Note that many of the components are under active development and continuous evolution. See roadmap for available components and future plan.