Ruler in a word file stays on the top and left to help you move table borders and line up objects. It is also very useful to measure stuff in the word document. The ruler by default is on when you open the word file, but just in case the ruler in your document disappears, please follow the steps below to bring it back.
Step 1: Click the "View" tab from the ribbon;
Step 2: Check the "Ruler" box in the "Show" area;
Step 3: The ruler will display in the word document.