Shards are in-game objects that are displayed as small fragments circling the top of a portal in the scanner. They usually appear as part of a shardgame-event.
The objective of the event is gather all shards to a target Portal, to complete the crystal. Completing them will yield some reward. They move along a Link in regular time intervals that depend on the current rules. Portals may need to be of a certain level for the shard to jump to them. They will randomly move if they stay on one portal for too long.