The beacons window allows you to locate certain types of objects, by identifying them visually on your screen. The window is activated via the top menu → World → Show More → Beacons, or by pressing Ctrl-Alt-Shift-N.
You can opt to show beacons to object, or have objects highlighted in red - or both. (Note that you cannot disable both selections; one will always toggle on.)
The slider underneath controls how wide the beacon lines will be. (It does not affect highlight mode.)
The check boxes below control what kinds of objects will be indicated:
You may select one or more of these to be highlighted on screen.
Beacons are handy for finding lost or “unknown” objects, or “griefer” prims.
If you wish to search for items by name or description, try Area Search.