The bookmakers overround is the profit margin of the bookmaker. Having an overround is how bookmakers make money. To explain this you need to understand the probabilities of events occurring. For example if there are 3 horses in a race the true probably of one of the three horses winning the race is 100% as one of the horses has to win the race. A bookmaker will set different odds (the probability of that horse winning) for each horse. The bookmaker will set odds that add up to more than 100%. Here is an example using decimal odds as it easier to explain.

If the bookmaker gives winning odds for three horses as follow.

Horse A has odds of 1.75
Horse B has odds of 2.30
Horse C has odds of 10.80

To add up the three probabilities you first convert them to percentages.

100/1.75=57.14
100/2.30=43.48
100/10.80=9.26

Adding the three together

57.14+43.48+9.26= 109.88

Therefore the bookmaker has worked out the total probability of one of the horses winning as 109.88%. So the bookmaker has factored in a profit margin of 9.88% on this race.

This information is useful to know because sometimes bookmakers make mistakes, and if after calculating a set of odds you reach a sum less than 100% the total odds work out in your favour rather than the bookmakers. Betting on an event like this is called a value bet.