"Never again" is a promise or a commitment not to let something bad happen again. It means that we will try our best to prevent a similar bad thing from happening again in the future.
For example, if someone gets bitten by a snake and ends up in the hospital, they might say "never again", which means they will try their best to avoid getting bitten by a snake in the future. They might take precautions such as wearing protective clothing or staying away from areas where snakes are likely to be found.
Similarly, if a person has experienced a traumatic event or witnessed a terrible tragedy, they might say "never again" to express their desire to prevent such events from happening again. This phrase is often used in the context of tragedies that have occurred in history, such as genocides or wars, where people make a vow to never let such horrors happen again.
In short, "never again" is a way of expressing a determination to prevent a bad thing from happening again, whether it is on a personal or a larger scale.