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.