Hellenismos is a very big word that means the way of life, culture, and religion followed by the ancient Greeks in the old days.
Long, long ago, before there were computers, phones or even cars, the Greeks had their own gods and goddesses whom they worshipped and believed in. They had big, fancy temples where they would offer prayers and sacrifices to their deities.
These gods and goddesses were believed to control the elements of life like the sun, the sea, and the earth. For example, Zeus-the King of the gods was thought to control thunder and lighting, and they believed in him in a religious manner.
Even though this practice of Greek religion is not as common today, people who follow the beliefs of Hellenismos still hold high regard for the ancient rituals and customs set forth. They practice meditation, honor their ancestors, and pay homage to the gods, especially during their festivals, by offering special prayers and other offerings.
In summary, Hellenismos is a way of living and religion that originated from Ancient Greece, revolving around the gods and goddesses worshipped by the Greeks at the time. Though it's not commonly practiced anymore, some people still honor the ancient customs and practices to this day.