The calculated attribute doesn't get stored in the database, instead every time you retrieve an object it gets calculated.
There are valid use case scenarios for calculated attributes, but generally I suggest you avoid calculated attributes as they can bog down your app easily when working with large sets of data.
Hope this helps.
A stored attribute is an attribute which value is stored in the database.
A calculated attribute (or Calculated Value) is an attribute which value is calculated EVERYTIME the object it is in, is used.
Have a look at the academy (use search in the top right), there are a few sections about the difference between calculated and stored attributes.