Note value referring to how long, how many beats, a note is held. But of course there’s always more to it than that.
Take the whole note, for example, held for a full measure (4 beats) in 4/4 time. The simplest note on paper—elegant, rounded, full of space. It looks empty sitting there on the staff, and when you are young it seems like a place to wait until you get to the more interesting notes. Soon you learn to count out the beats instead of merely waiting—1, 2, 3, 4. Later you learn to subdivide, one and two and three and four and, to give each beat its due.
It is quite a bit later that you learn to think of this as a note to shape, as sound that has dimension. After the beginning of the note (and how does it begin? with a consonant sound? a vowel sound?) it does not have to move through time completely flat, although it can if you choose. It can mimic its own shape, soft at the start, gently swelling through the middle, rounding-out to soft again at the end. It can start strong and fade away. It can start strong and grow louder, too, or fade and grow again, or start quiet and increase in volume all the way to the next note.
No matter how it is shaped, a whole note rarely stands alone. Even this takes time to learn. For years it is something you do automatically, but gradually you learn to be intentional about it: every note either moves toward or away from something. Each note has shape, yes, but each note is also part of some larger shape. The whole note seems to know this best. That time it requires from you can either help or hurt what you are trying to say with the music; phrases die within this note, or are sustained and strengthened and soar.
A whole note is rarely flashy. It does not dance or march or run or tumble. Often it accompanies the (faster) notes that do, offering support, or color, even sometimes a shimmer of silver from tremolo violins. But despite its non-flashiness, you would not want music without whole notes. You need that grounding for everything else—a canvas, a base color. Broad strokes that rise and fall slowly, like hills or plains or mountains. Not the opposite of movement, but a counterweight, something to hold against the sharpness of 16th notes, the fluttering of 32nds and 64ths. Something to ease the progress of halves and quarters as they move through their melody. A place to rest, or build, a chance to suspend time.
This is not a note to be gotten-through on your way to better things. This is a noble note, with noble things to show you.
You resolve to never overlook its wholeness.