SP stands for Starting Price.
It is the official price of a horse at the moment the race starts. If you place a bet at SP, you are accepting whatever the final starting price turns out to be rather than taking an earlier fixed price.
For example, if you back a horse at SP and it goes off at 4/1, your bet is settled at 4/1.
SP is useful for punters who do not want to take an early price, or who place bets very close to the off. It is also an important reference point when talking about whether a horse has shortened or drifted in the market.

