Gridlines are useful in Excel helping to specify each cell, by default, it appears when you open a workbook. However, if you want to hide them, please follow the steps below, :
Step 1: Click the "File" Tab from the ribbon;
Step 2: Click "Options" from the left navigation menu;
Step 3: Click "Advanced" from the left navigation menu in the dialog box;
Step 4: Uncheck "Show gridlines" under the section of "Display options for this worksheet";
Step 5: Click "OK" at the bottom.