What does betting across the board mean in horse racing?

Betting a horse across the board means to back it in the win, place and show placings.
In other words, if you fancy a horse named Can Do but aren’t sure he’s good enough to win, you might bet $10 on him in all three placings, figuring you will at least collect something if he runs well.
In general this isn’t a smart move, as you could wind up losing money even if Can Do runs third. If his odds are high enough, a show bet would or incorporating him in exactas or trifectas with logical favorites might lead to better results over the long run.