Count distinct, also sometimes referred to as 'count unique' is a function that allows you to count the distinct values in a particular column.

E.g. if you have a query with customers & reservations, you might want to know how many individual reservations were made. You can do this as follows:

1. Add a Summarize step, and click the + next to Metrics.
2. Hover over count and then scroll down to the section where it says 'Count distinct' 
3. Select the column that you want to get distinct values for 
   
   

Using 'count distinct' can be a great way to remove duplicates in your query, because you can just see how many unique values there are.