An alias is a reflection of a dashboard. It will always mirror the structure and reports of the dashboard. You can customise the alias by applying filters and new visualisations, but any changes to the report metrics and dimensions must be done at the master dashboard. This way all aliases always reflect the master setup.
This is useful when you have twenty markets to report. Instead of making 20 identical dashboards for each market, you make one dashboard and 20 aliases, filtered for each market. This would save a lot of time especially when editing the dashboard.
To make one simply go to the dashboard you would like an alias for and click the + and 'Alias'.
From your alias dashboard click again to the + to return to the master dashboard.