This term is often used when considering image acquisition through remote sensing and refers to the time of the year during which an image is taken. Leaf-on and leaf-off refer to the presence or lack of the foliage of woody species. Leaf-on imagery means that there is foliage on the tree or shrub species (or the species of interest). Leaf-off means that there is no foliage or a reduced amount of foliage on the tree or shrub species.
Sometimes it is beneficial to have leaf-off imagery so that you can see ground features more distinctly. This is helpful for mapping features such as buildings and roads, which may be obscured by tree foliage during the growing season. Leaf-off imagery is also used in forestry applications because the lack of leaves on some trees facilitates the classification of tree types.
There are times when you might want leaf-on imagery, especially if the tree or shrub species has a distinctive spectral reflectance that can be distinguished from other vegetation. Leaf-on imagery is also used in agricultural applications to measure the quantity and health of crops. Many woody species may have similar spectral reflectance or structure that may benefit from either a leaf-off or leaf-on flyover.