When I play an "A" and you play an "A" do they sound the same? Or is my "A" slightly higher (or slightly lower) than your "A"?
Most players play a little sharp (higher) on some notes, and a little lower (flat) on others. Or, if you don't have the mouthpiece set at the right place, all of your notes can be higher or lower than "normal."
BUT, when playing together, its important that everything is exactly together -- no one higher or lower than each other. That is being "in tune" (intonation) with the other players. Playing with good "intonation" means adjusting your pitch to match everyone else's.