Stake not returned (SNR) is a type of free bet offered by most bookmakers.
It means that the bookmaker will give you a free bet for a certain amount, say £20. You can use this to bet on whatever you like, however the initial £20 is not included in your winnings.
To use an example:
In a normal bet - When you bet £20 on a 10.0 (10/1) horse, you would make £200 including your initial deposit. The bookmaker would return to you £200.
In a free bet with "stake not returned" =When you bet £20 on a 10.0 (10/1) horse, you would make £180. As there is no initial deposit the bookmaker would simply return you £180