Plain English Meaning
The visibility is better than 6 miles. Forecasters don't bother giving an exact figure beyond that — anything above 6 miles is just reported as P6SM.
Full Definition
A contraction used in aviation weather reports and forecasts (such as METARs and TAFs) to indicate that prevailing visibility is greater than 6 statute miles. The 'P' stands for 'plus,' meaning 'more than,' and 6SM is the maximum reported value in U.S. surface visibility coding.
Context Anchor
Seen in the visibility part of TAF and METAR weather reports.
Derivation
The 'P' is shorthand for 'plus' (more than), and 'SM' stands for 'statute miles' — the standard unit of visibility in U.S. weather reports. Statute miles are the everyday land miles (5,280 feet), not nautical miles.