Much like Damage Reduction, Energy Resistance is a creature's resistance to a specific form of energy. The main types of energy to which a creature can be resistant to include acid, cold, fire and electricity. There is typically no way to overcome a creature's Energy Resistance aside from simply not using the form of energy they are resistant to.
Some races - like Aasimir - have natural Energy Resistance, but otherwise characters will have to rely on items or spells. Fortunately, enemies rarely employ a wide variety of elements against you, so spells like Resist Energy, Communal or Protection from Energy, Communal should be more than sufficient.
More uncommon is Energy Vulnerability, which indicates that a creature is especially susceptible damage from an energy source. A creature is either vulnerable to an element or not, and if they are they receive 50% more damage from that energy source. If a creature with Fire Vulnerability was hit with a Fireball, it would take 150% normal damage, for example.