Across describes something that's situated on the opposite side or the direction you have to go to get from one side to another.
Thinking about swimming across the English Channel? It's 23.7 miles across from England to France, so you might want to take a boat instead. The phrase "across the board" means that all parts of something are affected. But it originally came from horse racing, to describe when someone bet equal amounts of money on a horse to come in first, second, or third place. The "board" was the blackboard where bookmakers chalked up the odds for each horse.
