The term keywords is in a way synonymous to search. It helps a person be able to retrieve an Item or a group of Items based on the keywords he/she entered in the search bar. This so happens because someone (Person/Algorithm) has formerly associated those keywords with those Items. In technical terms it is known as Indexing.
With reference to documents, it is people who are working or collaborating with it who know the essential keywords for that given document. With SharePoint, we enable our end users to add keywords. Keywords can be added to 2 different types of Items. It can either be added to a list Item of a SharePoint List or it can be added to a document in a Document Library.
Obviously when you are trying to add an Item to a list or a document library, you would need to add it to a specific column. The column is called the “Enterprise Keywords” column. Over here, end users can add any number of keywords to any of the list Items (In case it is a SharePoint List) or a document (In case it is a Document Library).
In the figure below, you can probably see a clipping of a Document Library with columns- Modified By and Enterprise Keywords. In Enterprise Keywords, one would be able to specify the keywords to be associated with a document or a list Item.
In order to add the Enterprise Keywords Column to a content type, one would first have to configure the Default storage location for the keywords. The default location for the keywords should point to the Managed Meta Data Service. This can be done in the Central Administration.