Note Value is a fancy way of saying “note length”. It is how long or short a note is to be played and is expressed as a fraction.
The whole note is a good starting point. A whole note represents a certain duration of time. A half note is half the length of a whole note. A quarter note is a quarter the length of a whole note. A quarter is also half the length of a half note. The important thing to keep in mind is that the length of all these note values are in relation to one another.
If this way of thinking about time is challenging, please consider this: imagine a pizza. You buy a whole pizza. You take it home, and before you consume it, it is whole. If you cut the pizza in half, you have two halves. If you cut those halves in half, then you have four quarters.
The difference between note value and pizza is that note values represent a duration of time, whereas pizza is space.
Another important consideration is that, like pizza, it being whole has nothing to do with it being big or small–or in the case of music, long or short. Fast music has whole notes that go by quickly (short duration). Slow music has whole notes that go by slowly (long duration). Within any given tempo, however, a half note is still half the length of a whole note, and a quarter note is still a quarter the length of a whole note.