While the access tokens have short (1 hour) expiry time, the refresh tokens have an expiry of 50 years. When the Access token expires, you are able to use the latest Refresh token acquired to get a new set of access and refresh tokens.
Every new refresh token cancels the previous one.
Store the Refresh tokens safely, for example in a database or in the filesystem. Due to its long-lived nature, refresh tokens should always be stored safely. By saving them to permanent storage, you will also avoid having to enter your eAccounting user credentials, after for example a server reboot.