system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can instruments be in a key?

Background on me: I am a classically trained trombonist who struggles with music theory.

I know that a trombone is in the key of Bb, but what does that mean? The key is determined by the piece your playing? Additionally, a trombone with an F trigger is shifted into the key of F when the trigger is depressed (same with the G trigger on bass trombone). What does that mean? For me it just means that first position is now 6th.
gpt
With instruments it means that the notation is conventionally transposed.

For example: a piano is easy to play in C and music is easy to read in C.

However, a Bb trumpet is easiest to play in Bb. So music for it is commonly written in Bb. But Bb is more difficult to read, unless you enjoy lots of accidentals in the key signature. So the music as written as though it is in C, but C on the stave is sounded as a Bb, D is sounded as a C, and so on.

It therefore means that music in the easiest to read key is easiest to play.