What cents are, and why they matter when tuning
The unit any tuner uses to measure how in-tune a note is, explained simply.
What a cent is
A cent is one hundredth of a semitone. A hundred cents separate one note from the next (say, C to C#). It's the unit tuners use to measure how close or far a note is from being perfectly in tune — much finer than "a little sharp" or "a little flat" by ear.
Why a tuner shows cents, not just the note name
Knowing you're playing an "A" isn't enough — an A can sound 20 to 30 cents off and still be recognizable as an A to a trained ear, but it'll clash when played alongside other instruments. That's why the tuner shows both the note and the cents of deviation: the goal is landing within ±5 cents, roughly where most ears stop noticing a difference.
Why some strings are harder to tune
Lower strings (like a guitar's low E) have longer wave cycles, so the tuner needs to "listen" to more signal to calculate the frequency accurately — that's why it helps to let the note ring for a second or two instead of plucking and releasing it right away.