I need explanation on Civil Disobedience because I am unclear on it.

You might need to be a bit more specific as to where
exactly your lack of clarity lies.  Thoreau was a part of the Transcendentalist movement
in American History.  He believed that individuals needed to do a better job of hearing
their own hearts and passions in daily life.  The political extension of this is the
notion of civil disobedience, taking a passionate and personal stand when government or
society is wrong.  For Thoreau, if individuals do not actively speak out when they feel
government is wrong, they do not show the appropriate respect to their emotional frame
of reference.  Since Thoreau feels that individuals commitment and emotions are the most
important elements in one's state of being, to speak one's mind, regardless of the
consequences, is the highest and most transcendental form of being in the world
represented.  For example, when Thoreau disagrees with the Mexican- American War as a
form of political conquest and territorial expansion, he is willing to stand for his
beliefs, including going to jail in refusal to pay taxes to support the war.  This is an
example of the concept of civil disobedience and one that helps to bring out the meaning
of the work in full form.