Secular things are not religious. Anything not affiliated with a church or faith can be called secular.
Non-religious people can be called atheists or agnostics, but to describe things, activities, or attitudes that have nothing to do with religion, you can use the word secular. Public schools are secular, but Catholic schools are not. Grocery stores are secular; a synagogue is not. If there's no religion involved, then you're in "the secular world" — as people sometimes call everything that exists outside of religion.