It is a very important part of everyday life. It was one of the first things that blew me away with love when moving here, how open people were about speaking about their faith. It is beautifully liberating. Sri Lanka is a Buddhist country. Buddhism is the state religion and is practiced by the overwhelming majority. The country practices Theravada Buddhism, a school of Buddhist thought that developed in Sri Lanka before spreading to South East Asia. Including Buddhism, there are 4 major religions in Sri Lanka.
For the most part, these religions coexist harmoniously, with Colombo having an abundance of temples, mosques and churches.
On the whole, Sri Lankans are forgiving when it comes to foreigners not understanding how religious events work and will let you know if there is anything specific you ought to know. General rule of thumb is to dress modestly and carry a shawl and wear long trousers if you are attending a religious event and be respectful in your photography.