Use the login endpoint to generate your JWT bearer token. Pass in your username/password, click "Try It" in the grey box on the right, and copy the value within the quotes on line 5 (token) of the response block. It should be a very long string.
This is how you will authenticate all subsequent calls-- not your username/password. This token will be valid for 1 hour. After 1 hour, you will need to generate a new one for security reasons.
Let's use the List Meters endpoint as an example.
Paste the bearer token in the top right field of any call in the documentation and click "Try It". This will return the raw JSON data for the given request in the response section.
For List Meters, if you want to see a specific meter's data, you can add a filter in the Query Params section. Below is meterNumber=="123" for example. Please see this section for more details on searching. You can then analyze this data in the website itself or copy/paste into a tool like VS Code to see it more clearly.