a1 refers to an object of class Alpha.
a2 refers to another object of class Alpha.
Class Alpha has two member variables, b1 and b2.
a1.b1 refers to the member variable b1 in the Alpha object that a1 refers to.
a1.b2 refers to the member variable b2 in the Alpha object that a1 refers to.
a2.b2 refers to the member variable b2 in the Alpha object that a2 refers to.
These are fundamental concepts about classes. See
Classes in Oracle's
Java Tutorials to learn how classes work.