Used only in array or tabular environments.
The \multicolumn is command is used to make an entry that spans several columns.
It is useful for making table headings which span several columns, or for changing the positioning of the heading for a single column (the pos argument in \multicolumn overriding that of the \tabular value, for example).
The first mandatory argument, cols, specifies the number of columns to span. The second mandatory argument, pos, specifies the formatting of the entry;
\multicolumn must either begin a row or be placed immediately after an &.