Value in quilting is defined as the relative lightness or darkness of the color in a fabric when it is placed next to another colored fabric. Value is relative, which means it changes based on what colors are being compared.
For example, consider the following three blocks. In this example the medium blue would be considered low value.
However, notice in this grouping, the medium blue has high value, meaning it is more noticeable.
Another way of looking at it, is that high value colors tend to have more “pop” whereas low value colors tend to recede.