a very simple way of explaining it is the amount you would need to bet on the race in order to return £100 which ever horse wins.
eg 5 horse race each at 4/1 you would need to bet 20 on each,
therefore 100 bet and 100 returned this is a 100% book , if one horse was 3/1 this would then become a 105% book
the higher the figure the bigger the edge in the bookies favour!
hope this helps?