Tags are meant to describe specific details of your posts. Think of these as your site’s index words. 
These are popular keywords that you can use to loosely relate your posts. This makes it easy for users to find your posts.

For example, your categories can be something like: Healthy Baby, Healthy Beauty, Healthy Home etc. 
Now when you write a post about Toxic Chemicals in Popular Cleaning Products in the Market, you will categorize it under Healthy Home. 
You can add tags like Get Clean, toxic chemicals, cleaning products, etc.

When you compare it in a book, categories are the chapters and tags are the index.

You can add the tag while you are in edit mode. Scroll down to the bottom and on the side bar, look for "Tags".
Type the words you want to use as tags on the bar. Make sure that you separate each word with a comma. 
Click on "Add" when you are done.

Check the screenshot below.