An ocularum (Plural: oculara) is a magical object that appears as a skull with a crystal placed in the right eye socket through the back of the skull.
They are found throughout the world, usually placed at high elevations upon pedestals. Their purpose seems to be to reveal the locations of magical shards that were previously hidden until the Breach somehow disrupted the magic that had been hiding them from view.
Looking through the crystal in the skull reveals the glow of the shards, pinpointing their location. To find the shards, scroll around the view until a white circle surrounds the shard and a tone is heard. Complete this for however many shards are linked to the ocularum in question, which will be between three and eight shards. Each ocularum provides an approximately 180 degree view of the region forward from the position of the ocularum. It is almost always the case that the shards are evenly distributed in the field of view, so if there are four shards to find, one can expect to find them at every 45 degrees or so of yaw, though at varying pitches. Shards are never found in trees or at the 0 degree or 180 degrees of yaw.
Finding the first shard, regardless of which ocularum you use, will trigger the war table operation Investigate the Shards. Enchanter Renaud finds a scroll that makes mention of the shards, and after doing some research discovers that they may be able to unlock an elven temple in the Forbidden Oasis called Solasan. Collecting the shards enables the completion of several side quests (see Collections for more details), and once all shards have been collected the final door in the very heart of the temple can be opened.
See also EditCodex entry: Ocularum Codex entry: Oculara Codex entry: A Worn Diary Codex entry: Leather-Bound Hakkonite Journal