Idioms are expressions with special meaning that are different from the meaning of the individual words. For example, ‘to have your feet on the ground’ is an idiom meaning ‘to be sensible’ (Encarta Dictionary).
True story …. a former colleague recounted how when she first arrived in Australia she had been invited to a friend’s party and was asked to bring a plate. Bring a plate? ….. She thought may be her friend didn’t have enough plates. She went to the party and presented her empty plate and noticed the look of amusement on her friend’s face. Her friend kindly explained that to ‘bring a plate’ is an Australian idiom for bringing a plate of food for everyone to share.