You will win (or share in the case of multiple winners) the jackpot amount for that pool. You can remind yourself of the amount you are playing for by looking at your ticket under “My tickets”.
Remember that if you chose a lower stake than the default when you placed your bet, you are playing for a fraction of the headline jackpot. For example, in a 1X2 Pick 5 pool, a £2 stake per line plays for £5,000, a £0.50 stake per line plays for £1,250 and a £0.20 stake per line plays for £500.