This is an equation for calculating the porosity of a medium. The porosity percentage represents the relative amounts of space occupied by the "voids" in a volume of material, where the voids can be a combination of liquids or gases.
This porosity percentage is a measure of how compact the material is. A volume of large rocks will obviously have a great deal of space between them and thus have a large porosity percentage compared to fine particulate of a volume of a clay soil, which would have very limited amounts of voids.
The input to this equation is the void ratio, `e`.
Notes
In engineering application the porosity is used to indicate a tendency of a material to expand or contract. A high porosity percentage is likely to be seen in loose soils which then will contract under pressure or added loads. A low porosity percentage represents a material that is dense. This porosity percentage affects how fluids will flow through a medium or how particulates can move through the medium.