When you place a bet on a horse race, instead of taking the fixed odds price on offer you might be able to place a bet at an unspecified Starting Price. Sometimes for future races, which haven’t been priced up by your selected bookmaker, you can only place a bet at the Starting Price (SP).
When you place a bet at the SP, you do not know in advance what odds you are getting! So, why on earth would you want to take this option? Odds on horses can fluctuate, in the build up to the race. When the race start time comes closer, it is clearer what the conditions will be like (e.g. is the ground heavy or soft, will it be raining or really hot at the time of the race), if any other horses have withdrawn, and if the horse has any issues etc. Odds can thus fluctuate for many reasons, including the amount being bet on a horse. So, with lots of money coming in, the price is likely to go down and vice-versa. In the UK, a panel decides the starting price (it should be the same regardless of which bookmaker you use), based on how they view the fluctuations of prices.