/ (forward slash) in css style declarations

It's shorthand for font-size: 12px; line-height: 18px.

It simply means font-size and line-height

font: 12px/18px /*12px font-size and 18px line-height*/

That's a short-hand notation...There are many more in CSS which you can use, for example

margin-top: 10px;
margin-right: 20px;
margin-bottom: 30px;
margin-left: 40px;

Can be simply written as

margin: 10px 20px 30px 40px
         ^----^----^----^
       Top/Right/Bottom/Left

Or say for example this

border-width: 1px;
border-style:solid;
border-color: #ff0000;

Can be written as

border: 1px solid #f0000;

Here's a cool list of CSS shorthand.

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