Simply put, these are quantitative measurements. For example, the Sessions metric means the total number of sessions on the tracked website. The Average Time on Page metric is the number of seconds a user spends on a page during a single session.
In Google Analytics reports, metrics are combined with dimensions. In this case, dimensions serve as data attributes. For example, the dimension Country signifies the country (US, UK, France) where the traffic is coming from. The dimension Page is used to indicate the URL of the viewed web page.
Most Analytics reports display dimension values in rows, whereas metrics are put in columns. Here is an example of dimensions and metrics provided by Google Analytics. The dimension is an attribute of metrics put in rows (City). Metrics (Sessions and Pages/Session) are in the columns paid with dimensions.
What is a “metric” in Google Analytics? https://support.google.com/analytics/answer/1033861