This equation can calculate relative permittivity.  it assumes that the permittivity (`epsilon`) is measured in Farads per meter.  For some common dielectric constants, please see this Dielectric Constant lookup equation.

Brief Version

`epsilon_r` is inversely proportional to how much stronger the electric field is in a given material compared to vacuum.  

More

Polarization (`P`) is what happens when an electric field (`E`) pushes on a material where the charges can move a little, but aren't fully free.  You can think of it as the charges being shifted so that they may not cancel out in a given area.  However, the charge of the entire material is still conserved.

Linear Dielectrics¹ are materials that obey the equation `P = epsilon_0 chi_e E` for `E`s that are not too strong.   `chi_e` is called the "electric susceptibility", and it can be thought of as describing how far the charges are free to shift when the electric field pushes on them.  Linear Dielectrics simplify a lot of math and therefore can be quite useful, but that's outside the scope of this brief discussion.  For a more detailed account of dielectrics and polarization, please see The Physics Hypertextbook.

In Linear Dielectrics, the equation we discussed earlier eventually gives us the equation `epsilon = epsilon_r epsilon_0`, where `epsilon` describes  an electric field's strength in a given material.  Higher `epsilon` means a weaker field, and vice versa.  We can understand this in the context of polarization as `epsilon` representing how much the charges in the dielectric shift to counteract the electric field.  We can rearrange the above equation to .

`epsilon_0` is the "permittivity of vacuum", and can be thought of as describing the strength of an electric field when there isn't a dielectric around to become polarized and oppose the field.  `epsilon_r` is the ratio between the permittivity of a material and the permittivity of vacuum, so it's a ratio that describes how much weaker an electric field is in a given material relative to vacuum (hence Relative Permittivity).  For example, if `epsilon_r` for a given material is 2, then an electric field in that material will be half as strong as it would be in vacuum.

1. ^ Griffiths, David J. "Electric Fields in Matter." Introduction to Electrodynamics. 4th ed. N.p.: Prentice Hall, 2013. 185-86. Print.