Prezi provides a selection of premade icons and shapes that you can insert in your presentation to provide extra interest of emphasis. Click on the "Insert" button in the main menu and select "symbols & shapes" from the drop-down menu.
This will bring up a menu of shapes from which you can choose. Simply highlight the appropriate one and click "Choose" in the lower right of the menu. This will insert it into your presentation. Shapes can be scaled and moved in the same manner as any other element.
You can also draw lines with and without arrows by clicking on "Frames & Arrows in the main menu and selecting "Draw arrow" or "Draw line" from the drop-down menu.
Click and hold, moving your cursor in the direction you wish the line to go, and drag your mouse until the line reaches the desired length. You will also be able to edit these aspects after the fact (see this page).
Another drawing-related function is the "Highlighter" tool within the "Frames & Arrows" menu.
Click and drag your cursor to create this effect. If you move your cursor quickly, prezi will adjust your highlight to a straight line. Otherwise it will appear somewhat ragged to emulate a "hand-drawn" effect. And again, you can rescale, rotate, or reposition your highlight after you execute it (see this page). Note that in this case rescaling will both lengthen/shorten and widen/narrow your mark; you cannot alter these aspects individually.