Parilia is an ancient Roman holiday that was celebrated every year to honor the god of shepherds, named Pales. Imagine that Pales was a kind of imaginary friend who Roman shepherds believed helped them take care of their sheep and keep them safe.
During the celebration, shepherds would gather all their sheep in one place and perform a special ceremony to make sure that everything would stay good and happy for their sheep for the rest of the year. This ceremony involved a lot of special things like making a special cake called the "Palilia", which was made from milk, salt, and herbs, and a lot of special prayers, songs and dances.
People also believed that the smoke from burning herbs and laurel leaves would make the sheep feel good and keep them healthy. So they would light brightly colored candles and burn different plants.
So, basically, parilia was a very special day for Roman shepherds that they celebrated to show their appreciation for Pales, their imaginary friend, and to make sure their sheep would stay healthy and happy for the rest of the year.