Can’t understand $atts in functions?

In general, if arguments is not used or the defaults values are to used of an argument/parameter, you don’t have to write it out, they can simply be omitted. The only time when you have to pass any value to an argument is when a function expect a valid value to be passed to it in order to properly execute, something like get_terms() which expects the first argument to have a valid taxonomy value

In your example, using shortcodes, it is not necessary to pass $atts as you are not expecting any values from attributes and you have not set up any attributes inside your shortcode function. Your shortcode simply returns a static value. In a case like this, you can simply just omit $atts