Betting markets are all about percentages. Odds are just a representation of someone’s opinion of an event occurring. That opinion can be expressed as odds or as a percentage - as most non-gamblers would be familiar with.
There are ‘true’ odds which represent the exact probability of an event occurring (e.g. even money on Heads in a coin toss) and there are bookmakers’ odds (e.g. 1.9 on either side in a coin toss). A bookmaker adds percentage to his markets in order to turn a profit - this means his prices should be lower than the actual price of an event happening.
Bear in mind that only certain events can have their true odds calculated - the chance of heads on a coin toss, rolling a six or an Ace appearing on the river in poker. Calculating the exact odds in a horse race or sporting contest is impossible - there are far too many variables involved, thus it comes down to peoples’ opinions.
Think of a match odds market in football. A high-profile market on Betfair will always trade very close to 100%, the point where all the money going into the market equals all the money being paid out. Bookmakers run a business - they need to turn a profit, so they will price up a football match odds market to around 110%. This means for every £110 they take from punters as bets, they aim to only pay out £100. It doesn’t work quite that simply, they need to leave room for price movement and deal with taking more bets on one team - a 4% profit margin over a season on football match odds markets would usually be seen as a good year.
This means little to most Betfair customers as they are concerned with one or two runners, not the whole market.