When you want to apply some formatting method to a larger text fragment, you can select it. CKEditor supports a few selection methods that you can use.
Selecting All Document Contents
To select all contents of the document, click inside CKEditor editing area and press the 
toolbar button. All document elements, including text, images, or tables, will become highlighted. You can now apply whatever action you wish to the selected content.
Alternatively, you can also use the Ctrl+A keyboard shortcut to select everything inside the editing area.
Selecting Document Fragment
A popular way to select document fragments is to use your mouse. Keeping the left mouse button pressed, drag your mouse over the document content that you want to select. The selection process will finish, when you release the mouse button. A document fragment selected with your mouse will become highlighted.
Note that a document fragment can also be selected with your keyboard. Put the cursor at the start of the selection and press the Shift button. Navigate the document using the Arrow keys to select a fragment of your choice.