1) Any terms from the SharePoint Term Store. This includes any terms from any taxonomy/term set as well as any “Managed Keywords”.
2) If a document is checked into SharePoint that already has tags associated with it, then those tags will be added to the Enterprise Keywords field automatically at time of check in. Most documents added to SharePoint won't have this metadata (because most people don't take the time to add keyword metadata to documents), but some will. These keywords will then show up in the term store under “Managed Keywords”
3) If the term store administrator has made the “Managed Keywords” term set an open term set, then users can enter any keyword they wish into the Enterprise Keywords column. Management of these user generated tags should be part of your SharePoint governance process.
More information can be found at: