A "Twitter bomb" is when a lot of people on Twitter get together to post the same message at the same time to make it go viral. Think of it like everyone yelling the same thing at once, but instead of doing it in person, they do it online.
For example, if a group of people are upset about something, they might all use a specific hashtag and tweet the same message with it. This can make it seem like the message is really popular and important, even if it might not be true.
Sometimes, people use Twitter bombs to spread messages or start a movement for a cause, while other times it can be used to troll or harass someone. It's important to use social media responsibly and not be mean to others.