When someone secularises something, they make it not religious anymore.
Secularising something can be good or bad, depending on how you feel about religion.
Examples:
The government secularises the schools.
She secularises the holiday by not going to church.
He secularises the music by taking out the religious words.
The cat secularises the mouse by chasing it away.
The wizard secularises the magic by making it not about gods.
The king secularises the kingdom by making new rules.