​The main two things to set in Custom Reports are the Dimension and Metric.  It helps me to imagine the final table.  In a graph, the Metric would be the Y axis and the Dimension would be the X axis.  The Metric is dependent variable, the Dimension is an independent variable.

Whatever your first column in the table is, make that your Dimension.  Examples of dimensions are IP address, month, operating system, continent. It has to be something finite and listable.
Disallowed dimension: number of hits (not finite)
Allowed metric: browser (Firefox, Brave, Opera, et al)

Whatever you want to put in the 2nd column and beyond can be a metric.