A check is considered posted when it is included on the Post Checks And Print Check Register report, which you run after printing checks. When this report is run, it updates several Accounts Payable files (namely, the Open Item File, the Check Reconciliation File, and the A/P to G/L Distribution File).
A check is considered unposted when it has printed, but the Post Checks And Print Check Register report has not been run yet (and consequently, the Accounts Payable files have not yet been updated).