An across the board bet is simply a singular way of betting a horse to win, place, AND show. In reality, you’re placing three unique bets – one to win, one to place, and one to show – but the advantage to betting a horse across the board is that you don’t have to recite or input these bets individually, you just say you want X dollar amount ‘across the board’ on Y horse. The total cost of your bet will be three times the base amount, as you’re actually placing three different wagers.
An across the board bet usually isn’t a great idea (after all, a show bet is involved), but it does a nice job of reducing risk while still keeping the potential reward fairly high.
If you’re going to bet a horse across the board, just make sure that the win odds are appealing to you, because if you don’t believe there’s value in the win pool, then there’s very little chance the other pools will be any more favorable.