Explanation
Pretty common question on the Tableau Desktop Specialist exam.
1) First, lets create Sheet 1. For this, drag sport to the Row shelf, and Weight to the Text mark in the Marks shelf. Change its aggregation to Average:

2) Now, for sheet 2 - Drag nationality to the view, and gold to the size mark in the Marks shelf.
NOTE: Depending on your version of Tableau , you may need to assign a Geographical role to the nationality column first as follows:


3) Now, let's create a dashboard, and use both these sheets in it:

4) Now, for the most Important step, use SHEET 2 AS A FILTER FOR SHEET 1 as follows:

Now simply click on Russia in Sheet 2, and Sheet 1 will automatically update as follows:

Explanation
Distribution Pane
Explanation
This is the Analytics pane! Read more from the official documentation below:

Explanation
By default, measures placed in a view are aggregated by SUM, which means that the data for that field in all of the rows is combined. Measures can also be aggregated as average, median, count, or count distinct.