It is to left align elements on a page. The only parameter is the content to be aligned. See also Template:Align for more options and flexibility.

With no input arguments, it saves typing the following text (in a table):

style="text-align:left"|

To use, type {{left}} at the start of the table cell. It will leave Category:Pages using left with no arguments.

With an input argument, it saves typing:

<div style="float:left;">...</div>

To use, type {{left|content}} to align contents.