This is one of those signs that you should know about or you might trip over. In Thai it says "prot ra-wang peun dtaang ra-dap" which translates as "please be careful of the floor that has different levels". Or more simply, "mind the step".

โปรด ระวัง พื้น ต่าง ระดับ (prot ra-wang peun dtaang ra-dap)