Many search engine marketers think who you link out to (and who links to you) helps determine a topical community of sites in any field or a hub of authority. Quite simply, you want to be in that hub, at the centre if possible (however unlikely), but at least in it. I like to think of this one as a good thing to remember in the future as search engines get even better at determining topical relevancy of pages, but I have never actually seen any granular ranking benefit (for the page in question) from linking out.
Google and Bing use a crawler (Googlebot and Bingbot) that spiders the web looking for new links to find. These bots might find a link to your homepage somewhere on the web and then crawl and index the pages of your site if all your pages are linked together. If your website has an XML sitemap, for instance, Google will use that to include that content in its index. An XML sitemap is INCLUSIVE, not EXCLUSIVE. Google will crawl and index every single page on your site – even pages out with an XML sitemap.
Keyword research tells you what topics people care about and, assuming you use the right SEO tool, how popular those topics actually are among your audience. The operative term here is topics -- by researching keywords that are getting a high volume of searches per month, you can identify and sort your content into topics that you want to create content on. Then, you can use these topics to dictate which keywords you look for and target.
SEM is a broader term than SEO, and is used to encompass different options available to use a search engine’s technology, including paid ads. SEM is often used to describe acts associated with researching, submitting and positioning a website within search engines. It includes things such as search engine optimization, paid listings and other search-engine related services and functions that will increase exposure and traffic to your Web site.
An SEO meta description is a brief description of content found on a web page. The meta description is shown to users in search engine results pages to help them decide which search result to click on. Meta descriptions are not a ranking factor in the eyes of a search engine, but they can influence how many people click on a result -- which directly affects the result's organic ranking.
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.
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.