What does the forward slash mean in the CSS font shorthand?

12px is the font size, 18px is the line height.

The syntax is based on typographical notation for specifying the respective sizes, and is only applicable to the font shorthand property. In other words, the above declaration simply expands to the following:

font-size: 12px;
line-height: 18px;

As always, if you set the line height to a relative value (e.g. percentage or ems), it's calculated relative to the font size.

W3C CSS2.1 font property reference
W3C CSS3 Fonts Module font property reference (the syntax carries over from CSS2.1)

It's equivalent to:

font-size: 12px;
line-height: 18px;