Foliar chemistry represents the chemical content within the leaves of canopy. Chemical molecules can be chlorophyll, carotenoid, nitrogen, etc.
Change in chemical content may change with vegetation phenology and health. It can be directly measured through destructive methods or indirectly estimated from ground/airborne/satellite-based remote sensing (see here for ground measurements).
Whereas direct measurements of chemical content are in mol/m2, indirect estimations are unit-less indices.