Fluorine's electron configuration in the ground state is 1s2 2s2 2p5. This means that the electrons in the fluorine atom are in their lowest possible energy state. These electrons are held in orbitals in each energy level.
The Aufbau principle gives the reason why electrons stay in the ground state configuration. Electrons must fill the lowest energy levels first before spilling into the higher energy levels. Each energy level also contains orbitals, and the lower orbitals must fill before the higher ones. For example, s orbitals have to be filled on a certain energy level before the p orbitals on that same energy level can start filling.
Fluorine holds on to its electrons tightly. Being the most electronegative element on the periodic table, fluorine combines readily with other elements, accepting a single electron to fill its outermost energy level.