Disadvantage

Whenever a creature is hampered in some way, it may have one or more disadvantages when making attacks. Disadvantage represents a difficulty or hardship that prevents a creature from fully focusing on an attack.

When you have one or more disadvantages versus a creature, you suffer a penalty to attack rolls made against that creature. You suffer a -2 penalty for each disadvantage that you have (i.e. -2 for one disadvantage, -4 for two disadvantages, -6 for three disadvantages, etc).

Disadvantages are usually imposed by the following: Status Effects, Concealment, Cover, certain abilities, environmental effects, through certain powers and abilities, and any time the GM rules that a creature is unable to properly attack.

Advantage vs Disadvantage

Disadvantage is the opposite of Advantage, and each advantage and disadvantage cancel out on a 1 per 1 basis. If you have more advantages than disadvantages, then you still have advantage on your attack. Similarly, if you have more disadvantages than advantages, then you still have disadvantage on your attack. If the number of advantages and disadvantages are equal, then you have neither advantage nor disadvantage on the attack. For example, if you had 3 advantages, and 1 disadvantage, then you are left with 2 advantages (and get a +4 bonus to the attack roll). If you had 2 advantages and 2 disadvantages, then they both cancel out and you have neither advantage or disadvantage (and you have no bonus or penalty to the attack roll). If you have 1 advantage and 2 disadvantages, then you have 1 disadvantage (and suffer a -2 penalty to the attack). If a power requires you to have advantage, then the requirement is not met unless you have more advantages than disadvantages and vice versa.