You have the option of marking entries. Keyboard shortcuts are Ctrl + M and Ctrl + Shift + M to mark and unmark the selected entries, respectively, and you can also access these actions in the menu Edit or by right-clicking on the entry and selecting the relevant action.
Marking is not the same as selecting entries. Marked entries behave differently from other entries in two ways. First, they are always displayed with a different background color in the entry table. Second, they will float to the top of your entry list, as long as your table is not sorted according to groups.
The color each marked entry is displayed in depends on the marking level of the entry, with the default colors ranging from yellow to red. There are menu choices for directly setting the marking to a specific level.
Marking entries can be useful for instance if you need to do several searches without losing your first search results, or if you need to remember which articles or books to look more closely at.
In addition to manually marking entries, JabRef can optionally auto-mark the latest imported entries, and at the same time unmark the previous entries marked in this way. Such entries will be shown in a different color from manually marked entries - by default green. This feature is toggled under Options -> Preferences -> General: ‘Mark entries imported into an existing database’ and ‘Unmark all entries before importing new entries into an existing database’.
Marking of entries will be associated with your username, the same name as is used to fill in the owner field of entries you create or import (this will by default match your user name in the operating system, but can be changed under Options -> Preferences -> General). Different users can mark entries independently. Saving the database will preserve the markings.