What is that companies do that promise to improve a person's Web site ranking?

I was contacted by someone who promised to improve the ranking of my Web site. I do not make money from the site. It is just a collection of pages related to high school level math. I told the guy that I was definitely not interested in a monthly plan and he said he could offer a single month’s worth of work. I asked him specifically what he would do. He said something about good keywords in metatags and something about social media recognition.

I am very skeptical about what can be done. Do these companies actually do anything useful?