Hi there,
This is not a stupid question! For someone who plays a concert pitch instrument, the idea of transposing instruments is very confusing.
Basically, all “C” instruments like guitar, piano, strings, flute, oboe, etc. will sound the same pitch when they play A=440.
There are a few different transposing instrument families:
Bb instruments include clarinet and trumpet.
Eb instruments include Alto Saxophone and Eb Clarinet (the smaller one).
F instruments include the French horn and English horn.
When you play C on the Bb trumpet, it sounds the same as a Bb on the piano. When you play C on an Eb saxophone, it’s the same pitch as Eb on the piano. Etc, etc.
I hope this is helpful!