Question 1

What is ROI?

Accepted Answer

Return on investment (ROI) measures the gain or loss generated on an investment relative to the amount of money invested. ROI is usually expressed as a percentage. 100% means no gain no loss while anything above 100% shows that you've earned more than you've spent.

Question 2

How do you calculate ROI?

Accepted Answer

ROI is a ratio of the "earned" money to the "spent" money: ROI = Net Profit / Total Investment * 100. So to calculate your ROI you take all your income and divide it by the expenses. 
For example in sales it might be the ration of the Live Time Value (LTV) of each client to the total salary of sales team who were able to bring those clients.

Question 3

How do I improve my ROI?

Accepted Answer

Basically, improving ROI comes down to the two main factors: increasing LTV of your clients and/or decreasing the cost of acquiring each clients. The latter could be done for example by improving or semi-automating the sales process