Both come from the device information recorded with each open/click event:

• Reading environment is a doughnut chart splitting events into Webmail (opened in a browser-based mail client), Mobile and Desktop, as percentages. When no data exists yet the chart shows three equal placeholder slices.
• Top email clients lists the mail client/browser names detected and the percentage of recipients using each, sorted ascending.

Use them to decide how much to optimise for mobile (most food-delivery audiences read on phones) and which clients to test your design in. The values are approximate because some clients hide or rewrite this information.