The numbers 300,400... in the font-weight property indicate the weight of the font. Here is the complete list: 100 - thin 200 - extra light 300 - light 400 - normal 500 - medium 600 - semibold 700 - bold 800 - extra bold 900 - black
Browsers on the web display fonts slightly differently than they might look in, say, Photoshop. They usually come out a little bolder. Sometimes it's helpful to know the little tricks: you can specify, for example, a Light weight file to get a Normal font rendering.
And vice versa. If you don't have a Semi-bold (600) file, specify Bold instead. That way, semi-bold headings will be displayed in bold instead of the regular body text style.