The Calibrate button is incase you want to tune to a pitch other than concert... Sit in with an old concertena or vintage small pipes sometime... You'll see what I mean...

Have them play an A (or whatever note you wanna start tuning from depending on the make of your tuner, read the instruction book for more details) and have your tuner calibrate itself to that specific pitch... then tune yer axe with it...

I've done the same thing with bugger'd penny-whistles... a mate of mine had one with a realy hard 'ding' in the side of it... it went more than a few cents sharp and would never come back... so I'd plug into my tuner, have him play the whistel into the body-mic of the guitar with the strings muted, calibrate my tuner from that signal, then tune the guitar off that setting...

*shakes his fist at the tallented people who CAN tune by ear!*

;-)