You pay each time a user clicks on an SEM result. You pay nothing when a user clicks on an SEO result. SEM results are paid placements, and your brand is charged each time a user clicks on the result. Therefore, you must have a budget for continually showing SEM ads and using this form of PPC lead generation. On the flip side, you are never charged when a user clicks on an organic search result.
The SEO starter guide describes much of what your SEO will do for you. Although you don't need to know this guide well yourself if you're hiring a professional to do the work for you, it is useful to be familiar with these techniques, so that you can be aware if an SEO wants to use a technique that is not recommended or, worse, strongly discouraged.