In order to use specific font-weight, your font must support it, if it doesn't, then any value between 0 and 600 (not included, normal value is 400) will be interpreted as normal, and any greater value will be bold (bold normally is 700).
If your font doesn't have a light/thin variant, then I'm afraid you can't get a thinner font weight than normal (400).
EDIT NOTE : For fonts than only are normal (400), then bold is generated by default by the browser, for instance :
@import url('https://fonts.googleapis.com/css?family=Roboto');
p {
font-family: 'Roboto';
font-weight: 700;
}
<p>This is bold, but I didn't loaded Roboto 700, only Roboto 400.</p>
In this case, the render may differ from one browser to another, although it usually don't.