Well, it’s not a stupid question at all…
wp_title is a template tag and it was used for a long time (since 1.0.0) to put <title> tag in the header. This way plugins were able to modify the title with filters. In WP 4.4 it was about to be deprecated, but that idea was abandoned and you can still use that function.
On the other hand, you don’t have to. Since WP 4.1 you can use title-tag theme feature instead.
So if your theme declares that it supports title-tag feature, then the title tag will be generated automatically and you don’t have to put it manually in the header any more. And to declare that you’re supporting such feature you need something like this in your code:
function theme_slug_setup() {
add_theme_support( 'title-tag' );
}
add_action( 'after_setup_theme', 'theme_slug_setup' );