This is the default font family for all legend items in the chart. The default value for this property is Arial, but it can be any other Power BI supported font family:

Arial, Arial Black, Arial Unicode MS, Calibri, Cambria, Cambria Math, Candara, Comic Sans MS, Consolas, Constantia, Corbel, Courier New, Georgia, Lucida Sans Unicode, Segoe (Bold), Segoe UI, Segoe UI Light, Tahoma, Times New Roman, Trebuchet MS, Verdana, Symbol, Wingdings.