There are a lot of definitions of SEO (spelled Search engine optimisation in the UK, Australia and New Zealand, or search engine optimization in the United States and Canada) but organic SEO in 2020 is still mostly about getting free traffic from Google, the most popular search engine in the world (and almost the only game in town in the UK in 2020):
After a while, Google will know about your pages, and keep the ones it deems ‘useful’ – pages with original content, or pages with a lot of links to them. The rest will be de-indexed. Be careful – too many low-quality pages on your site will impact your overall site performance in Google. Google is on record talking about good and bad ratios of quality content to low-quality content.
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.
QUOTE: “The average duration metric for the particular group of resources can be a statistical measure computed from a data set of measurements of a length of time that elapses between a time that a given user clicks on a search result included in a search results web page that identifies a resource in the particular group of resources and a time that the given user navigates back to the search results web page. …Thus, the user experience can be improved because search results higher in the presentation order will better match the user’s informational needs.” High Quality Search Results based on Repeat Clicks and Visit Duration
Ask for a technical and search audit for your site to learn what they think needs to be done, why, and what the expected outcome should be. You'll probably have to pay for this. You will probably have to give them read-only access to your site on Search Console. (At this stage, don't grant them write access.) Your prospective SEO should be able to give you realistic estimates of improvement, and an estimate of the work involved. If they guarantee you that their changes will give you first place in search results, find someone else.