The Scope tab allows you to restrict the report to specific accounts, campaigns or ad groups. Selecting an Ad Account will cause a Campaigns tab to appear with the campaigns in that ad account. Likewise, selecting a campaign will cause an Ad Groups tab to appear.
Selecting an item on the left side, then clicking the [ >> ] button will add it to the scope list on the right. The report will be restricted to the combination of items selected. Note that selecting a combination of items of different levels (account, campaign, ad group) may yield unexpected results.