When you say that someone is the 'New Kid on the Block' you mean that they are a new arrival to the area or to the social group.
Example of use: “Jeff’s the new kid on the block; he doesn’t know all the rules yet.”
The phrase 'new kid on the block' originated in the United States and was first seen in print in a “Skippy “ cartoon by Percy Crosby: “The new kid on the block told me that the next time he saw you he was goin’ to twist your nose.”