SEO
Work on a site and its content so it appears for searches that bring you customers. Not a service with an end date, but upkeep.
Why it matters when you read an offer
The word appears in every second offer and rarely means the same thing twice. When you see it, do not ask whether SEO is included; ask what exactly will change on the site each month and what the result will be reported on.
What it looks like in an offer
An offer line like included SEO optimisation in a website build usually means filled-in titles and descriptions, not the monthly service from this page. Ask which of the two they mean.