It means that if there is zero activity on the account, I would assume it is not logged in/actively being used.
If you are using them, I wouldn't worry.
It does say this in the email
"Samsung defines account usage/activity as follows:
Creating a Samsung accountLogging in to Samsung services/products using a Samsung accountUsing a service/product while logged in with a Samsung account
To prevent your account from being deleted, and to ensure proper use of Samsung Services, your account must have at least one usage/activity every twenty-four (24) months.
However, there are some exceptions to this policy and accounts that meet at least one of the exceptions will be considered active.
Exceptions include, registered family accounts, accounts with a record of accumulating/using reward points, and accounts used to purchase products on Samsung.com. You can see the complete list of exceptions here.
Notifications will be sent to your email address before your account is deleted to prevent unwanted deletion.
In addition, we are making a change to how we notify you about updates to our Terms and Conditions. Moving forwards, we will notify you via email only when material changes are made, those that directly affect your rights and obligations"
It does say any enquiries reach out
https://www.samsung.com/uk/support/