It is important to ensure you use consistent journal names in your bibliography or reference list. Some referencing styles require journals to be listed with their full title, and others require the abbreviated title e.g. J Biol Chem.
To make it easier to manage journal names and to generate correct citations, Mendeley Desktop uses an abbreviated journal title if that is required by the rules of the style you have chosen.
The software does this by using a built-in list of journal names and associated abbreviations. This is generated automatically by the software and should cover most publications.
If the auto-generated abbreviations do not match the output that is required then you can edit the abbreviations in Mendeley Desktop.