A Word text comprises visible and invisible symbols. Visible symbols are the letters you type. Invisible symbols are for instance the paragraph marks, the tabulation, the space, just to name a few. Hiding invisible symbols facilitates reading. However, showing them will help correcting formatting mistakes.
In the screen capture above, you see tabulation (the arrow at the beginning), spaces (the dot between words) and the paragraph mark. To show or hide those symbols, you just have to click on the Show/Hide button in the Home menu (¶) or type Ctrl+*.