It means that the trumpet in question is "tunned" to Bb.
So, when you play notes without pushing any valve, it should play:
Bb - Fundamental
F - Bb's perfect fifth.
High D - major third
High F - perfect fifth
High Ab - Minor seventh
If I'm correct, the trumpet is a transposing instrument and even though the real pitch that sounds when you play is in concert Bb, it is written as C in the score.
So, if you'd like the trumpet to play a real G, in a score it would be written as an A.
Sheet music for a C mayor scale, would be written in a D Major scale. That's at least what I learned.
The simplest way to understand it is, you hear a Bb, and you read as C and finger it thinking pressing no valves plays a C. So I suppose this is useful for when switching different instruments of the same family with different tunings, so that fingering always adjusts to the same notes.
If I'm wrong in any aspect may anyone correct me.
You hear Bb
and write and read as C