It means private message. If you are a VIP member, you can send private messages back and forth to fellow hobbyists and providers. A "private" venue to discuss whatever you wish to.
 
Or if you want the other definition

The 12-hour clock is a timekeeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (a.m., from Latin, meaning "before mid day", or "before the middle of the day") and post meridiem (p.m., "after mid day" or "after the middle of the day"). Each period consists of 12 hours numbered 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11.[1] Twelve o'clock is either noon or midnight.


You decide which one is appropriate in this particular venue