The cursor is a small, blinking vertical line; you can make it appear in a "Name" box, a "Command" box, or a "Mod" box by clicking inside the box. The cursor tells the computer where to put the symbols you type or the calculator operations you select.
If there is text inside a text box (i.e. the "Command" box or the "Name" box), you can select it by pointing to the text, holding the mouse-button down, and dragging the mouse along the text.
In order to view symbols at the beginning of the text, you can click in the text, keep the mouse-button down, and drag the mouse to the left, slightly beyond the left boundary of the box. You can similarly view symbols at the end of the text by dragging to the right. Another way to do the same thing is to just repeatedly hit the left arrow or right arrow on the keyboard.
In order to remove the symbol to the left of the cursor, use the "delete" key on the keyboard.