Search engine marketing (SEM) is different approach of getting your website to search results. Instead of on/off-site optimizations, you pay directly to get your site into search results. If you are familiar with something like that:

then you already know what SEM is. Those results marked Ad are not organic results. In fact, for competitive keywords like above, you will not even see organic results above the fold.

What is the advantage?

The speed. SEM will get you to the top spot very quickly, so you can offer your products and services to motivated buyers immediately after airing your ads.

SEO takes time, SEM is almost immediate.

There is a catch though. It may be pricey and for competitive keywords it surely will. So it's always good to mix up your efforts in SEM with old-school SEO, because the efforts put in your site, backlinks and reputation will last. Traffic from paid ads are gone the second you turn them off.