Pay Per Click (or PPC for short) is the act of paying for advertising space on search engines and other promotional channels.
A good example would be Google Ads. Google Ads allow you to pay to have a page on your website appear at the top.
While this isn't strictly SEO, the impact is the same, providing an immediate result. Whereas SEO takes time to see results, paid advertising is there and then.
A side effect of paid advertising is that it is driving lots more high quality traffic to your website. Traffic is a SEO signal and once you have engaged with people previously not aware of your business, they are more likely to return without seeing a paid advert.