Font weights come in two styles: a numeric value like “400”, and a short phrase like “normal”. The range of numeric font weights is easy to understand–at the low end they start at “100”, which would be the thinnest/lightest version of a font, and they go up by 100s, ending at “900” for the thickest/blackest version.
What is more difficult to understand is what textual font weight corresponds with its numeric equivalent. Above, we used an example where 400 is “normal”. That convention is more-or-less cast in stone, although you will find exceptions without trying too hard. The other common one is 700 being equivalent to “bold”.
Not many people know what the other weights are called, and more importantly, what order they are in in terms of lightest to blackest. Here’s the reference you need to decode that:
200: Light or Thin
300: Book or Demi
400: Normal or Regular
600: Semibold, Demibold
800: Black, Extra Bold or Heavy
900: Extra Black, Fat, Poster or Ultra Black