Seo stands for search engine optimization. That's a big fance title for someone who ensures that your website ranks in the top ten in the google search results. Technically a search engine optimization specialist will rank your website in all of the search engines and not just google. But the truth of the matter is that Google is the only one that really counts right now.
What I mean by "Rank" is that your website shows up on page one (the top ten results) when someone does a search query related to your site. So if you own a flower delivery company and you are based out of the Boston MA area, then you would want to rank highly for searches like "flower delivery boston" and related searches.
Online marketing is a simple concept in that your website needs traffic for you to convert sales. That's it! Nothing more. Of course it's is not easy to do but that doesn't keep the concept from being simple. How well does your traffic convert to sales? Well that's another question entirely. But the fact is you can't convert any traffic at all if you don't have some to begin with.
The seo is responsible for driving quality traffic to your site by way of search engine surfers, a term which is totally dead by the way. The main point is knowing what a search engine optimization specialist can do for you and that is drive traffic to your site. Some companies prefer a search engine optimization specialist who doubles as a marketing consultant. These kind of seo's will do two things. On the one hand they will drive traffic to your site and on the other hand they will refine your site so that it converts well.
That is not strictly seo though and those guys charge big bucks indeed. Strictly speaking an seo specialist will drive quality traffic. Now there are some things to look out for. If your seo is charging you a mint to add your meta tags to your website get another seo! This kind of seo is not the real thing! Most seo is done off site by way of strategic linking and network building. So if your new seo guy is charging you an arm and a leg to add titles to your pages, h1 tags and meta tags run for the hills.
In my experience a good seo makes no promises so stay away from seo's that guarantee anything. There are no guarantees in seo for we can't predict how google will react to seo every single time. So no promises. Your seo should keep track of your rankings though and the traffic being driven. Beware, a spike in traffic doesn't mean much if the traffic is not quality traffic. Quality traffic comes from the search engines and not social media sites.
Keep in mind that seo's usually need a few months for their work to start showing some results. Results typically take 2 months to start taking effect. Slowly but surely is the way. Increasing your ranking dramatically in a short time may set off a flag at google, which you don't want.