A rather confusing term often informally used to mean 'in tune' or 'in tune with others'.
What it actually means is that a standard absolute reference pitch is used as the basis for tuning so that all pitches are standard rather than relative to each other.
The normal reference is A440 meaning A (above middle C) has a pitch of 440 Hz and this shall be true for all Instruments that play together under Concert Pitch. This removes any vagueries or ambiguities and also provides a standard for 'concert' instrument makers.
Not really used in Folk Music because Folk Instruments don't all lend themselves to tuning to Concert Pitch, however, Most Reed and Woodwind Instruments can be manufactured now so that their tuning lies within Concert Pitch and String Instruments can be easily tuned to Concert. Electronic Tuners use A440 as the normal reference but most allow it to be altered if needs be.