@Felthry This is a CSS rule. CSS is laid out roughly like so:
<element restriction> {
<thing to change>: <change> <modifier>;
}
In this case, it's saying
<All elements> {
<font boldness>: <standard> !<even if later rules try to make something bold or thin>;
}
Basically, that rule makes it so that bold tags like <b> or <strong> just simply don't work the way they're supposed to