This is often used in front of a person's first name when addressing them. In Thai, surnames are almost never used in speech. Instead, a person's first name is used together with "Mr.; Mrs.; Miss". For example, John Smith would in English be called "Mr. Smith", since Smith is the surname. In Thai however, they would perfectly correctly and politely be called "Mr. John", not "Mr. Smith".
Sometimes when talking about foreigners they will say "Mr. Smith" rather than "Mr. John", but in that case they would instead use มิสเตอร์, Mr. (loanword).