Before Reg FD it was normal for management to guide selected analysts privately. The rule requires that if material information is disclosed intentionally to market professionals or holders likely to trade, it must be released publicly at the same time; an unintentional slip must be cured promptly, usually within a business day.
Compliance produced the scripted earnings call, the pre-announced conference webcast, and the form-8-k filing of guidance. Quiet periods around results exist because managers would rather say nothing than test the line.
It does not prohibit analysts from assembling a mosaic of non-material pieces into a valuable conclusion, and it does not itself create insider-trading liability. Enforcement targets the issuer and its people, not the recipient.
Related: material-non-public-information, insider-trading, form-8-k, earnings-call, sec