ROI

Roi means "a king". It also has various sets of meaning, including Republic of Ireland (ROI) and Return of Investment. It simply measures the success or failure of an investment.

Relating to the internet, the ROI is very important. Unfortunately, many web site owners do not think in this way or have no interest in using the web to create more turnover or profit. They simple have a homepage because everybody else does it and "it might be convenient". Thus the homepage is like the company's identity card. Nothing more.

However, a web site can be a source of income. The first step should be getting through to people, i.e. having guest visiting the page. ROI is thus connected with SEO and scoring well on Google and the other Search Engines.

Wikipedia discusses ROI in the following manner:

In finance, rate of return (ROR) or return on investment (ROI), or sometimes just return, is the ratio of money gained or lost on an investment relative to the
amount of money invested. The amount of money gained or lost may be referred to
as interest, profit/loss, gain/loss, or net income/loss. The money invested may be referred to as the asset, capital, principal, or the cost basis of the investment. ROI is usually given as a percent rather than decimal value.
ROI is also known as rate of profit.
ROI does not indicate how long an investment is held. However, ROI is most often stated as an annual or annualized rate of return, and it is most often stated for a calendar or fiscal year. In this article, “ROI” indicates an annual or annualized rate of return, unless otherwise noted.
ROI is used to compare returns on investments where the money gained or lost — or the money invested — are not easily compared using monetary values. For instance, a $1,000 investment that earns $50 in interest generates more cash than a $100 investment that earns $20 in interest, but the $100 investment earns a higher return on investment.
$50/$1,000 = 5% ROI
$20/$100 = 20% ROI

Ok. If you invest some 10.000 USD in out website you should like to regain at least some of that money, if not making a profit.

That is what SEO helps you to do, and ROI helps you to understand.