Shortcodes in WordPress are bits of text you can use in the content area to invoke some kind of function to accomplish certain tasks.

To use a shortcode from within a template instead of with the content of a Post/Page, you can invoke it with a special function:

<?php echo do_shortcode("[shortcode]"); ?>

This is an easy way to make all of my existing blog posts full-width by default. All you have to do is add this code to your Theme Options in the custom css section:

.single-post .container:before {
display: none!important;
}
.single-post #left-area {
width: 100% !important;
}

.search-results .container:before {
display: none!important;
}
.search-results  #left-area {
width: 100% !important;
}

.search-no-results .container:before {
display: none!important;
}
.search-no-results  #left-area {
width: 100% !important;
}

.archive .container:before {
display: none!important;
}
.archive  #left-area {
width: 100% !important;
}

.page .container:before {
display: none!important;
}
.page #left-area {
width: 100% !important;
}