Bugs can come from code, data, configuration, assumptions, or timing. Good bug reports include steps to reproduce, expected behavior, actual behavior, and evidence.
ELI5
Think of Bug as a named building block in a big LEGO app. A mistake in software that causes incorrect or unexpected behavior.
In practice
Use it when you are mapping how the app is structured or explaining a feature to a teammate or agent. In practice, define the owner, input, output, and failure mode before you rely on it.