SEA stands for Search Engine Advertising. This is the paid side of search engine marketing where you buy traffic for certain relevant search terms. You will get the best results if you also have your SEO in order. This is because the quality score of a page is a big factor in how much you will pay for a click.
For example, if you sell bicycles but there is a lot of traffic on the keyword buy motorcycle then you could also buy traffic on that with the hope that those people would also consider a bicycle. However, because your page is about bicycles and not motorcycles, a search engine will recognize this and will charge you (much) more or simply not show it because your page is not relevant enough for the search.
So with only SEA, you usually won't manage to run a successful campaign.