To access the 'Overview Tab', go to 'Marketing Funnels' and click 'Funnel' that you want to overview.
The first tab that you will be presented with is an 'Overview Tab'.
This will give you an overview of 'Activities' that have happened inside of your funnel including the number of 'Visitors', 'Leads' that you've created & 'Visitor - Lead Conversion' rate.
Other activities can be found in the box on the right-hand-side.
Also, watch the video on 'An Overview of Marketing Funnels'