Roasted is when food is cooked over low heat for a long time, usually making it taste really good.
When people say "I love roasted chicken", they often mean that they enjoy eating chicken that's been slow-cooked until it's tender and flavorful. You might also hear someone ask if you want your vegetables roasted in the oven, which means they're asking if you prefer them to be cooked slowly in a dry heat. Some people use roasting as a way to bring out the natural flavors of ingredients - for example, roasting nuts can make them smell amazing and taste better too.
How common is "roasted"?
Word roasted is considered uncommon in modern English. It appears most frequently in magazine texts and less often in other writings.
Definitions
verb
To cook food by heating in an oven or over a fire without covering, resulting in a crisp, possibly even slightly charred appearance.
Example: to roast meat on a spit
To cook by surrounding with hot embers, ashes, sand, etc.
Example: to roast a potato in ashes
To process by drying through exposure to sun or artificial heat
Example: Coffee beans need roasting before use.
To heat to excess; to heat violently; to burn.
To admonish someone vigorously
Example: I’m late home for the fourth time this week; my mate will really roast me this time.
To subject to bantering, severely criticize, sometimes as a comedy routine.
Example: The class clown enjoys being roasted by mates as well as staff.
To dissipate by heat the volatile parts of, as ores.