Concert A (A4 in scientific pitch notation) is a pitch that is an octave above the A string on your cello.

Concert A is, however, the pitch of the violin A string, and will be what the concertmaster plays when the orchestra tunes. If you are in a full orchestra, you will most likely tune to this pitch given by the principal oboist. Don't let the different timbres of the instruments throw you off.
This pitch is also 440 Hz on a note generator. Many metronomes and tuners have note generators with this specific pitch to help you tune. Tone generators are also available online, though every musician should have a metronome as part of their kit.
Keep in mind that different orchestras may set concert A to a different pitch - for example, 442 Hz is common in some parts of Europe.
 Bow it softly but smoothly. Adjust the fine tuner (or peg, if it's really out of tune) until it is in tune.

If your arms are long enough, you can adjust the fine tuner with your left hand while simultaneously bowing. This takes practice to sustain the smoothness of the bowing.
To play concert A, simply play that harmonic. Find the pitch on the A string (3rd finger 5th position), and instead of pressing all the way, lightly touch the string. The sound will be lighter and more airy, but you may be able to hear the intonation better.
Listen to a concert A.
Play your A string.
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