The Overview report, located at the top left of the screen, shows you the overall trend of clicks that your website received from the Google search results page.
The Performance report allow you to analyse the performance of the different keywords that people used to find your website.
You can select Average Click Through Rate and Average position to see the trend and this two KPIs for every single keyword.
You can also see the trend in time of every single keyword by clicking on it. Then click on the x remove the keyword you have selected.
Then click on the Pages tab to see the how the pages of your website are performing. Finally click on a page url in the report to see the performance in time of a single page. You should expect the average position to grow if you have done SEO optimization on a specific page