Simply put, you pay a platform, usually a search engine, such as Google, and they'll place your site as one of the first results for any of the keywords you chose. Paying only when the link is clicked, hence the name, pay-per-click or PPC for short.
When should I use PPC advertising?
PPC is best used when your goal is getting some 'hot' traffic, after all, most people will land on your website via a search engine query, and given that users are specifically searching for something relevant to you and PPC puts you in the forefront of Google, it's a ready-made sale in your hands. So stop dilly-dallying and schedule a call already.