Token authentication is a string that if provided by a client, allows it to connect. It is the most straightforward authentication provided by the NATS server.
To use token authentication, you can specify an authorization section with the token property set:
authorization {
token: "s3cr3t"
Token authentication can be used in the authorization section for clients and clusters.
Or start the server with the --auth flag:
nats-server --auth s3cr3t
A client can easily connect by specifying the server URL:
nats sub -s nats://s3cr3t@localhost:4222 ">"

Bcrypted Tokens

Tokens can be bcrypted enabling an additional layer of security, as the clear-text version of the token would not be persisted on the server configuration file.
You can generate bcrypted tokens and passwords using the nats tool:
nats server passwd
? Enter password [? for help] **********************
? Reenter password [? for help] **********************
Here's a simple configuration file:
authorization {
token: "$2a$11$PWIFAL8RsWyGI3jVZtO9Nu8.6jOxzxfZo7c/W0eLk017hjgUKWrhy"
The client will still require the clear-text token to connect:
nats sub -s nats://dag0HTXl4RGg7dXdaJwbC8@localhost:4222 ">"