Substituents or groups on a benzene ring can donate or withdraw electron density from the aromatic ring. This donation/withdrawal can be described by resonance and inductive effects.
This is the result of differences in electronegativity or polarizability.
Inductive Withdrawers - A Fluorine atom is more electronegative than a C atom. A fluorine atom withdraws electron density by way of induction from the benzene ring. This induction occurs through a sigma bond. Fluorine is an EWG as are most halogens and as such they would deactivate the ring (slow down the rate of an EAS reaction).
Inductive Donator - On the other hand a -CH3 donates electron density because of hyperconjugation and the polarity of the (3) C-H bonds. A methyl group or any alkyl group is a slight EDG (Electron Donating Group) and slightly activates the ring making it a better nucleophile and speed any EAS reaction with it.
This effect results from conjugation of either a lone pair of electrons or a π bond. The push or pull of electrons can be seen with resonance structures.
Resonance Donators - Resonance donators have a lone pair of electrons in conjugation with the ring. For example, phenol has the following resonance structures. In general, the halogens (I, Br, Cl F) and -OH, -NH2, NHR, NR2 will behave as a resonance donator. Except for the halogens they are all considered EDG's and activate the aromatic ring making them more nucleophilic. Notice how the curved arrows show the electrons being pushed into the ring. This activates it and speeds up EAS reactions.
Resonance Withdrawers - Resonance withdrawers have a pi bond in conjugation with the aromatic ring. For example, acetophenone has the following resonance structures. In general -NO2, ketone, aldehyde, nitriles behave as resonance withdrawers and deactivate the aromatic ring in EAS reactions. Notice how the curved arrows show the electrons being pulled out of the ring and a net positive charge left on the ring. This deactivates it and slows down EAS reaction.