What is the difference between a Metric, and a Dimension in Looker Studio?

Typically, people use campaign name, date, city, landing page etc, as the dimension.

Dimensions are most commonly used as non-numerical values.

Metrics provide you with a numeric value to that dimension. A metric could be # of impressions, or conversions, or total cost.

Combining both dimensions and metrics will provide you with meaningful data that can be visualized in different forms.