What does this sign mean?

This is a slow vehicle sign. This sign will be affixed to the rear of any vehicle that uses the road below 25 mph. For example, you may see this sign attached to a piece of farm equipment or mobile machinery. The orange center of the sign will be highly visible during the day, while the red outer triangle will be highly reflective at night. Always be ready to slow down upon approaching such a sign. [Slow Moving Vehicle Sign, Traffic Signs, Rules of the Road, Washington Driver Guide]