The Preview allows you to see what your email looks like, and by default you can find the Preview to the right of the code Editor. The Preview window has two classes of previews: Live and Inbox.
Live previews are the default Preview type. The default Live preview view is the Browser, which is an adjustable 500px width preview found to the right of the Editor. There are several other Live Preview views, which display the email in the dimensions of common devices, and a resizable preview, which can be set to any size. Live previews update automatically as you write code and are free.
Inbox Previews show you how your email appears in a real inbox on a real device. You can choose from a long list of potential devices/apps/clients to get Inbox Previews for.
Inbox Previews only update when you ask them to - each will be noted with a yellow banner when it is out of sync with the code in the editor. See the Pricing page for the cost of each inbox preview.
You can apply simulated visual impairments to any preview (Live or Inbox) either individually or all at once. Live previews have additional preview controls (block images, light/dark mode). You can take a screenshot of any preview (or previews) and save it to your computer.