A Fill layer is a type of layer that project a texture directly onto the mesh based on a specific mode. This type of layer avoid to manually paint the textures on the mesh. The fill layer can be edited via the Properties window.
The properties are divided in two categories : Fill properties and Material.
The Fill properties controls how the Material is applied and/or projected onto the mesh. The projection mode can be changed via the Projection dropdown.
The current projection modes available are :
Material (and Grayscale)
The Material / Grayscale controls are identical to the other tools. See the Paint tool documentation for more details.