The overview gives you a great snapshot of the people coming to your website, with Age in a column chart on the left and a pie chart showing gender on the right.
The yellow warning that says: "Some data in this report may have been removed when a threshold was applied" is a sign that you have reached a level of granularity in which individuals may be easier to identify, therefore Google Analytics has removed some of the data to ensure it is high-level and cannot be used to identify an individual.
The percentage of data used is shown in each box in the top corner to help you understand the size of your dataset. This can happen for Age, Gender, and Interest Reports and any report or segment that uses these.
Additionally, keep an eye out for the sampling warning in the top right. The example above shows 100 percent but sometimes you will reach sampling limits so the data will be slightly less accurate.
From the Overview page, you can change the little drop-down from Sessions to four additional on page metrics: