what is the meaning of 24hour clock

Twenty-Four Hour Clock  is where the time start from 00:00 AM and ends at 23:59 PM whereas in 12- hour clock the time start from 00:00 AM and ends at 11:59 PM​.
Examples:

8 hours after midnight is written as 08:00 (which is equal to 8:00 AM), or simply 0800, 

18 hours and 34 minutes after midnight is written as 18:34 (which is equal to 6:34 PM),