For our purposes, the half-value layer (HVL) is the thickness of material needed to reduce an x-ray beam to half its intensity. This value is determined by two factors: 1) Energy of the beam and 2) Material used.
Energy: As beam energy increases so does the penetration of the beam (i.e., more photons make it through the material). Thus, generally, increasing beam energy will also increase the HVL. For example, the tissue HVL at radiograph energies (~3cm) is ~3x greater than the HVL at mammography energies (~1cm).
Material: As a material increases in atomic number (Z) and density, more photons interact and thus less make it through the material. Thus, generally, higher Z materials and denser materials have a smaller HVL. For example, at diagnostic energies, the HVL for Lead is ~0.1mm, for Aluminum is ~7mm, and for Tissue is ~30mm (3cm).