What does this sign mean?

It's a temporary flag ("slow board").  There are four types in use.  They vary in appearance from one railroad to another, but the colors are the same on all railroads.

1. A Red/Yellow board alerts the crew that there will be a
2. red board two miles farther down the track.  The red board protects men and equipment working on (or adjacent to) the track.  There is normally a Track Bulletin Form B that is given to the crew at the start of their trip, giving the exact milepost limits of the restricted area, and naming the employee in charge of the work area.  A crew member must contact the employee in charge by radio and receive instructions for movement through the work area (or to stop and remain stopped until the employee in charge allows the train to pass through the limits.
3. A yellow board indicates that there is a temporary speed restriction two miles further down the track.  The crew receives a TrackBulletin Form A, which gives the limits and the maximum speed.  If the crew does not hold a Form A for that location, then, beginning two miles after passing the yellow board, the train must slow to 10 MPH and not increase speed for two additional miles or until a green board has been passed by the last car of the train.
4. A green board indicates the end of the temporary restriction and, after the last car of the train has passed the green board, speed may again be governed by timetable instructions and signal rules.