How to call a function in templates in Magento?

Question

Magento

Tried to read. I didn’t understand anything, some xml, something needs to be written somewhere.
I feel like a comic book character .
For example, I have a function:

function helloWorld() {
        return 'hello world';
}

What do I need to do to contact her in temlets?
You can show it step by step.

Answer 1

Where is your function?