Normally a bookmaker will be winner no matter on result. To do so they need bets that allow them to make money on each bet they offer. That’s why bookmakers set odds not based on probability but incorporating a margin so they can earn on every bet.
Lets return to the coin example, with £100
If a person stakes £100 on head and another stakes £100 on tail, no matter result, bookmaker will have to pay £100 which leaves them with no profit.
To gain profit bookmaker lowers price, so instead they pay £90 which means they offer odd 1.9 instead of 2.0, like this when two stakes of £100 bookmaker will pay only £90 and having a profit of £10. This is the bookmaker margin.
How to beat the Margin
If you win 2 out of 4 bets with odds 2.0 and without bookmakers margin you would be break even.
But if bookmaker takes its margin we will have 2*1.9 a return of 3.8 which means a loss of 5% on total investment.
Beating the margin means a success in long term betting to beat the bookmaker margin.