bug label is applied to an issue when the maintainer recognizes it as a genuine bug.
This is a good thing if you reported the issue, as it suggests the maintainer intends to fix it.
What is a bug?
It's not always clear what a bug is. In general, we define a bug as behavior contrary to the documentation (where documentation includes comments, typing information, in addition to written text). In other words, if you are using code in the way it was intended, and it isn't behaving in the way it was described, then it is likely a bug.
Performance issues can be sometimes be considered a bug. i.e. if code takes much longer or uses more system resources that it should reasonably be considered to require.