To the average coffee drinker, balance may seem like a bit of a black box. If this is you, just think about it like a two-sided scale. You won’t them both to be even and balance together. This is the same when drinking coffee. If a coffee is too bitter or too astringent, it will just seem ‘off’.
The coffee flavor wheel can help understand balance. If a particular coffee is not overloaded in one area, the coffee might be considered balanced.