The plus(+) powers mean either hyperopia (which is shorter length of eyeball than normal length)
Minus(-) powers mean that your eyeball is longer than normal (myopia).
If a person has -0.5 refers to the power of the lens. It implies you are very slightly nearsighted (it's difficult for you to focus on things that are far away); this is known as myopia.
otherwise if you have +0.5 It implies you are very slightly farsighted.