What is data visualisation in the context of business intelligence?
Data visualisation in any context is the art and science of organising data in such a way that makes it easily consumable by a human audience. Specifically, in the context of business intelligence, you are typically presenting facts and insights about a business to stakeholders in order for them to take action and make better decisions to run that business. 

Good data visualisation combines user experience design with an understanding of how people interpret data to provide something intuitive and compelling for the intended audience to use. 

Here are some examples of design and interpretation choices you might make:

In terms of design, many cultures write on a page from top to bottom, left to right. This means that the first place on any visualisation that a person from any of these cultures will look is top left. Consequently, the content you put in the top left space of your dashboard should be a critically important part of the story you are trying to tell. In a culture that writes right to left you might put higher-value content on the right-hand side of the visualisation. 

In terms of in terms interpretation you want to key into things that people are naturally good at interpreting. For example, if you are presenting sales by product category and you have 10 product categories you could choose to leverage a bar chart where the length of the bar represents sales or a pie chart where the angle of the slice represents sales. A bar chart would be a better choice because people find it easy to compare the length of a bar. We cannot distinguish between the sizes of a slice with anywhere near the same level of accuracy. Choosing a pie chart would leave your users spending more time and effort interpreting your data visualisation than if you had chosen a bar chart to present the information to them.