Straight from the horse's mouth means information that comes directly from the person who knows it best or from the original source. Because it comes from the source itself, people believe it is accurate and trustworthy. The expression is thought to come from horse racing. Experienced buyers could tell a horse's real age by looking at its teeth, so learning the truth directly from the horse was the most reliable way to know.
I heard straight from the horse's mouth that the shop will reopen next week.