Can you explain this examples

Let A be a subset of R and let p eA. Then p is called an interior point of A is there exists an &-nbd of p contained in A, i.e., if there exists an e> 0 such that ] p - e, p-e [cA. The set of all interior points of A is called the interior of A and is denoted by A° or by int A. A point p is called an Exterior point of A if there exists an &-nbd of p contained in the Complement A of A. The set of all Exterior points of A is called Exterior of A and is denoted by Ext A. A point p is called a boundary point (or frontier point) of a if it is neither an interior nor an Exterior nor an Exterior point of A. The set of all boundary points of A called the boundar (or frontier) of A and is denoted by b (A) [or Fr(A)]