An Each-Way bet is technically two bets, one for your horse to win and one for your horse to finish in one of the top places (typically the top five for the National, although some bookies will pay out for six). Half your stake goes on each bet, and your 'place' bet will pay out a quarter the odds of a win.
If you bet £1 each way (a £2 stake) on a horse at odds of 100-1, £1 of your stake will go on the horse to win, and £1 on your horse to place.
Win Prize = £100 Place Prize = £25
If the horse wins, you'll win both bets (£125 in this example). If it places 2nd - 5th you only win the second bet (£25).
TL;DR - You get money if your horse finishes in the top 5.