{two-column} is about the \twocolumn command and the class option of the same name. For questions about the {multicol} package, use the appropriate tag instead.
A lot of document classes, like article, report and book, provide for a two column layout using the class option twocolumn. There are also the commands \twocolumn and \onecolumn that can be used to switch to a two- or single-column layout within a document.
The dimensions \columnsep and
\columnseprule can be modified to set the space between the columns and the width of the rule separating the columns.
In a two-column layout, the floating environments table* and figure*, being alternate versions of the environments table and figure, spread over both columns.
