This is your Public-API-Key, which Mettl uses to identify your account.
This is the current UNIX or Epoch Timestamp (the number of seconds between 1970-01-01 00:00:00 and current UTC Time). Any current generated Timestamp will be valid for 24 hours only.
This is a unique Signature which has to be generated for each API request.
The Signature is generated by creating a String-to-Sign and hashing it with your Private-API-Key using the HMAC-SHA256 (for version v2 and v3) or HMAC-SHA1 (for v1) hashing algorithm with Base-64 and URL encoding.
Read more about the above Signature Generation Process in API Authentication and Signature Generation.