To make something not religious anymore, or to make it more connected to the normal world and less connected to religion.
Secularised can make people think of change, modern life, or less tradition.
Examples:
The government secularised the schools.
The new laws secularised the old customs.
The old church was secularised and became a library.
The cat secularised the mouse's house and made it a cat house.
The wizard secularised the magic book and made it a normal book.
She secularised the old dress and made it into a new one.