Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Summary
The HTML <multicol> element was an experimental element designed to allow multi-column layouts. It never got any significant traction and is not implemented in any major browsers.
Do not use this! In order to implement a multi-column layout, you should be using the regular HTML elements, like <div> in conjunction with the CSS Column feature.
Note that up to Firefox 22, though not supported, a <multicol> element was associated with an HTMLSpanElement. It was then fixed and is now associationd with a HTMLUnknownElement as requested by the specification.