What does FragmentManager and FragmentTransaction exactly do?

by 
getFragmentManager()

Return the FragmentManager for interacting with fragments associated
with this activity.

FragmentManager which is used to create transactions for adding, removing or replacing fragments.

fragmentManager.beginTransaction();

Start a series of edit operations on the Fragments associated with
this FragmentManager.

The FragmentTransaction object which will be used.

fragmentTransaction.replace(R.id.fragment_container, mFeedFragment);

Replaces the current fragment with the mFeedFragment on the layout with the id: R.id.fragment_container

fragmentTransaction.addToBackStack(null);

Add this transaction to the back stack. This means that the
transaction will be remembered after it is committed, and will reverse
its operation when later popped off the stack.

Useful for the return button usage so the transaction can be rolled back.
The parameter name:

Is an optional name for this back stack state, or null.

See for information the other question What is the meaning of addToBackStack with null parameter?

The Last statement commits the transaction and executes all commands.

See the google documentation for more help:

http://developer.android.com/reference/android/support/v4/app/FragmentActivity.html
http://developer.android.com/reference/android/app/FragmentManager.html
http://developer.android.com/reference/android/app/FragmentTransaction.html

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