JSON Web Tokens (JWTs) are cryptographically signed tokens that serve as a compact, secure way to represent users or sessions in web applications, including those built with Next.js. They are structured into three segments—header, claims, and signature—each base64 encoded and separated by periods. JWTs can be signed using either symmetric or asymmetric cryptography, with the latter offering greater security through the use of public and private keys. Verification involves checking the signature using a shared secret or public key, ensuring the token's authenticity and the trustworthiness of the encoded claims. In Next.js applications, JWTs can be transmitted via cookies or request headers, such as the Authorization header, and verified using libraries like jose. While cookies are often considered the most secure method due to their resistance to JavaScript access, headers offer cross-domain transmission capabilities. The article also highlights Clerk, a tool that simplifies JWT management by providing additional features like social sign-in, multi-tenancy, and billing solutions, allowing developers to focus more on product development.