Service to retrieve bearer access token.
Tokens expire 7 days after they are issued. When a token expires, request a new one with your original credentials.
Best practice to improve your application performance is to store access token to pass with each API request, and refresh only when expired.
This will significantly improve your application performance and latency versus retrieving token before each API call.

For Store API request make sure to also enable request credentials, to pass cookie, this is required to handle customer session.