Sai Tai Mai is Bangkok’s Southern Bus terminal. This means that all the buses in this terminal are south bound ( Hat Yai, Pattani, Yala, Narathiwat and etc). This actually quite a new terminal ( to me) because the terminal I’ve been to, like more that 10 years ago, was in a different location.
As for my standards, this bus terminal is quite good. It has an airconditioned mini shopping complex inside with several food outlets, shops and clean toilets. If you have a lot of waiting time, then I suggest that you just hang out at the main building first before going to the platforms. I find the platforms to be crowded ( with not enough seats) and noisy with the buses going in and out.
And by the way, if you plan to buy a bus ticket a few days ahead of your trip, you don’t have to go through the hassle of going to the terminal but can buy your tickets online through Thai Ticket Major).