There are 24 hours in a day, but usually times only go up to 12. After 11:00 is 12:00, and after 12:00 we go back to 1:00. Because this means there can only be 12 different times, we say that some times are a.m. (in the morning) and others are p.m. (afternoon or night). If you have absolutely no idea what time it is and someone says it's 8:00, you need to know whether that's a.m. or p.m. in order to know what time it actually is.
The other way to talk about times is to go from 0:00 (midnight) to 23:00 (11:00 p.m.). This way, the time always means "hours since midnight" -- if you say times this way, you don't need a.m. and p.m. For example, if you have no idea what time it is and someone says it's 13:00, then you know it's 13 hours after midnight, which can only mean 1:00 p.m. (in the afternoon).
14 00 hours is the same as saying 14:00, which means 14 hours since midnight. If you see a time that's more than 12:00, you can just subtract 12 from it and make it p.m. This means that 14 00 hours is the same as 2:00 p.m.