The Tooltip Format String will denote how data will be appeared when you hover your cursor over a chart component. For example, if a bar in a bar chart has a value of $4,000, hovering your cursor over the bar will display its amount. In addition, the Tooltip Format String can also determine how data will be presented. For example, a Tooltip Format String of {0:C2} will display all tooltips as a currency rounded to two decimal points. More information on Format String types can be found here.