Continuity mistake: During the election meeting held at the saloon, there's a large sign that reads "Capitol City Here We Come" on the wall by the stairs. We see this sign in numerous shots. When Valance enters the saloon there's still a sign on that wall by the stairs, but now the painted handwriting on this sign is different. Moreover, the original sign that had been by the stairs is now up on the side wall of the saloon, where Valance and his thugs stand; note the handwriting. (01:09:20 - 01:13:45)
Answer: At first glance, it looks like the word "LEFT" in large, widely-spaced lettering. It's very unlikely that this would be a blocking cue, telling Jimmy Stewart to move left (which he does from that point). That's not the way blocking cues are done, for one thing, and a seasoned actor such as Jimmy Stewart wouldn't need such a cue. It's also very unlikely that it's a warning message from John Wayne who is hiding in an alley across the street, on Stewart's right. John Wayne didn't want anyone to know that he actually killed Valance, so he wouldn't alert Jimmy Stewart with any messages scrawled in the street. All hypothesis aside, it's probably just footprints in the dirt, an illusion of light and shadow.