They work similar to a scatter chart, but bubble chart also features a third variable. The size of the data points visualize this third attribute.
A bubble chart requires at least one X Axis Field, one Y Axis Field and one Size Field. You can add more than one data to the Y Axis Field to create multiple series.
Alternatively, you can select a Group to separate the Y Axis data accordingly.
Please note that X Axis Field, Y Axis Field and Size Field need to contain numeric fields and remember to select an Expression for Y Axis Field and Size Field by clicking on the field first and then choosing from the drop down.