what does enot mean

ε₀ is the permittivity of free space. You can see its value in the book.

This factor is important if the intervening medium between the charges changes. for instance if the charges are picked up from air and placed at the same distance in water. In this case the factor that changes is the permittivity of the medium. This factor reduces the force between the charges. It becomes 80 times less than that in air as the permittivity of water is 80 times the permittivity of air.