This is the default font family for all legends in the chart. The default value for this property is Arial, but you can choose any other font family supported by Power BI, including:

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.