To erase an object,
(i) Start the ERASE command by choosing
(ii) Home tab > Modify panel > Erase tool. (or) Type ERASE (or) E in the Command line. Now the cursor changes from the crosshairs to a little square.
(iii) This is called the pick box.
(iv) Its appearance indicates that AutoCAD is ready for you to select objects on the screen. Also note the command line;  it's prompting you to select objects.

By pressing ESC while an operation is in progress, it terminates the command.

(i) The Close option is used to join the current point to the start point of the first line when two or more continuous lines are drawn.
(ii) If you specify the end point by using the mouse, then click at the start point of the first line or enter C at the Command prompt, as given in the Command prompt below.
Command: LINE
Specify first point: Pick the first point.
Specify next point or [Undo]: Pick the second point.
Specify next point or [Undo]: Pick the third point.
Specify next point or [Close/Undo]: C (The fourth point joins with the first point).

you can turn it off with the UCSICON command. Type the command name, and then enter the Off option, as follows:
(i) Command: ucsicon (Press ENTER.)
(ii) Enter an option [ON/OFF/All/No origin/Origin/Properties ]: off (Press ENTER.)

To hide all the buttons on the Ribbon the panel back to its respective tab by clicking the small button at the top right side.