Okay kiddo, let me explain synechococcus to you very simply. Synechococcus is a type of microbe - a tiny living thing that you cannot see with your naked eyes. It is special because it is a type of cyanobacteria that can make its own food using the sun's energy in a process called photosynthesis, just like plants do!
Synechococcus lives in water, particularly in oceans, where it floats around using a special layer called a capsule to protect itself from the water. It also has something called pigments that give it a greenish-blue color - this is why when you look at pictures of the ocean, you might see patches of green or blue, and those patches are sometimes synechococcus!
But synechococcus is not just any ordinary microbe. It plays a very important role in keeping the ocean healthy by converting carbon dioxide into oxygen, which other living things in the ocean, including humans, need to survive. It is also a source of food for many other marine creatures, like zooplankton which are tiny animals that also swim around in the water.
So, in summary, synechococcus is a tiny living thing in the ocean that is greenish-blue in color, makes its own food using the sun, and helps keep the ocean healthy by producing oxygen and being a source of food for other creatures.