Western people use a 24-hours system or an AM/PM system. Some people in Thailand will use the Western system but the most Thai will use a system where the 24-hours of a day are divided in 5 parts.
There are After midnight, Morning, Midday, Afternoon and Evening. To say "6 pm" you say "6 in the afternoon" but to say "7 pm" you say "1 in the evening". That could be confusing!
Have a look at Telling Thai Time where it is better explained and with some updates to it. Or have a look at Thai Time
But ..... Some people start every part (or some parts) with counting at 1 again. So "9 am" could be called "3 in the morning". That makes it even more confusing! So now you can read Thai time .... now still learning the way Thai people handle time and appointments. Good luck!