If somebody is said to be once bitten twice shy, it means that someone who has been hurt or who has had something go wrong will be far more careful the next time. It’s something that you say when you have had an unpleasant experience and are going to be more careful to avoid a similar experience in the future.
A neutralization reaction is the reaction between an acid and a base to produce water and a salt. A salt is another name for an ionic compound and the specific salt produced depends on the acid and base involved. The product solution is neutral while the starting solutions are acidic and basic respectively.