A dimension that can help you analyze site performance.
The dates in your date range.
A segment of data separated out in a report for comparison.
The numbers in a data set often paired with dimensions.
Explanation: Metrics are quantitative measurements. The metric Sessions is the total number of sessions. The metric Pages/Session is the average number of pages viewed per session.
Every report in Analytics is made up of dimensions and metrics.
Dimensions are attributes of your data. For example, the dimension City indicates the city, for example, “Paris” or “New York”, from which a session originates. The dimension Page indicates the URL of a page that is viewed.
Metrics are quantitative measurements. The metric Sessions is the total number of sessions. The metric Pages/Session is the average number of pages viewed per session.
The tables in most Analytics reports organize dimension values into rows, and metrics into columns. For example, this table shows one dimension (City) and two metrics (Sessions and Pages/Session).
In most Analytics reports, you can change the dimension and/or add a secondary dimension. For example, adding a Browser as a secondary dimension to the above table would result in the following:
Valid dimension-metric combinations
Not every metric can be combined with every dimension. Each dimension and metric has a scope: user-level, session-level, or hit-level. In most cases, it only makes sense to combine dimensions and metrics that share the same scope. For example, Sessions is a session-based metric so it can only be used with session-level dimensions like Source or City. It would not be logical to combine Sessions with a hit-level dimension like Page…………Read more here: https://support.google.com/analytics/answer/1033861