If you fall in love with a girl and the girl ditches you, what would your reaction be? You would probably be really angry, but at the same time you would be scared to fall in love again. Having had a bad experience previously, you would be reluctant to put yourself in the same situation again. That’s what the idiom, “once bitten twice shy”, means.
E.g. *The caterer arrived three hours late last time, so Rama is not hiring him again. I guess it’s a case of once bitten twice shy.
*The last time he ate fish, he almost choked. He’s stopped eating fish altogether — once bitten twice shy.