Reddit generates a set of resized previews for every image that is uploaded, at several widths, and serves whichever one suits the context. The feed uses a small one so scrolling stays fast; opening a post loads something larger. None of these is the file that was actually submitted. When you right-click and save, the browser writes out whatever it already loaded, which is a preview.
For viewing on a phone that difference is invisible. For anything where detail matters — printing, cropping, using an image as a reference — it is significant, because you are working from a copy that was already reduced. Requesting the original from i.redd.it avoids that entirely. Nothing is processed or enhanced; it is simply a request for the file that was already sitting there.
Worth being honest about the ceiling: Reddit does not upscale. If someone uploaded a 600-pixel image, the original is 600 pixels and no tool can produce more. Plenty of Reddit images were themselves saved from somewhere else at low resolution before being posted. We show the real dimensions before you download so you can judge whether a post is worth saving.
Galleries are the other thing worth knowing. Reddit added multi-image posts holding up to twenty pictures under a single URL, and the interface shows them one at a time behind arrows. Saving from the browser gets you exactly the image currently on screen. Pasting the post link here returns the whole set, which is usually what you wanted. Crossposts add one more wrinkle — the media lives on the original submission rather than the crosspost, so the link has to be followed back before anything can be fetched.