Real-world Information
Krill (order Euphausiacea) are tiny, shrimp-like marine crustaceans that form an essential part of the oceanic food web. Krill feed on phytoplankton, and they themselves become food for countless different aquatic animals such as whales (particularly baleen whales), seals, penguins, squid and fish. Krill are found in all of the world's oceans, but are most noticeable in the Southern Ocean, where the Antarctic krill (Euphausia superba) has an estimated biomass of around 379,000,000 tons (each individual weighs around 2 grams).
Krill are fished by humans, with the majority of krill fishing occurring in the Southern Ocean and around Japan (globally, around 150,000 to 200,000 tons a year). Most of the catch is used for aquaculture feed, as fish bait, or in the pharmaceutical industry, although Krill are also used for human consumption in Spain and parts of East Asia.
In-game Information
In Jurassicraft, Krill (upcoming in update 2.1.0) are used to feed the Coelacanth (and eventually also other (typically small) aquatic creatures). In order to obtain Krill, right-click on water with a Fine Net until a Krill item goes to your inventory. Plankton can also be obtained in the same way.
Once you have a Krill item, right-click with it on the water surface (like a lily pad) to place a Krill Swarm. The swarm will automatically dispense new Krill items every 5 minutes or so. It is recommended that you place several of these in a pool or tank containing Coelacanths, to keep them reliably fed.
Left-click on the swarm to break it and obtain the Krill item back into your inventory.