What is num?

In the fifth part of the iterators lesson, the following code is used as an explanation -
const jumbledNums = [123, 25, 78, 5, 9];

const lessThanTen = jumbledNums.findIndex(num => {
return num < 10;
How can this program run if ‘num’ is not defined ? I know that it will run but I can’t understand how.

num is a parameter, parameters are “placeholders” until the function is called.

findIndex will call the callback function for each value in the list and pass the value to the parameter

Thanks for helping !