The goal of successful SEO is to obtain a high-ranking placement in the search results page of a search engine (e.g. Google, Bing, Yahoo and other search engines). Internet users often do not click through pages and pages of search results, so where a site ranks in a search results page is essential for directing more traffic toward the site. The higher a website naturally ranks in organic results of a search, the greater the chance that that site will be visited by a user.
Being ‘relevant’ comes down to keywords & key phrases – in domain names, URLs, Title Elements, the number of times they are repeated in text on the page, text in image alt tags, rich markup and importantly in keyword links to the page in question. If you are relying on manipulating hidden elements on a page to do well in Google, you’ll probably trigger spam filters. If it is ‘hidden’ in on-page elements – beware relying on it too much to improve your rankings.
A website or URL’s ranking for keywords or keyword combinations varies from search engine to search engine. A domain may rank for a certain keyword in the top 3 on Bing, but not even be on the first page of the Google search results for the same keyword. Of course, the same is true of all search engines – Bing, Google, Yahoo and every other search engine uses its own method for calculating rankings and therefore ranks websites differently.
Repeat this exercise for as many topic buckets as you have. And remember, if you're having trouble coming up with relevant search terms, you can always head on over to your customer-facing colleagues -- those who are in Sales or Service -- and ask them what types of terms their prospects and customers use, or common questions they have. Those are often great starting points for keyword research.
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.