Where do I see what changed?
What's new, at the foot of the sidebar. It lists what shipped, newest first, grouped by month, with each release dated. A dot appears beside it when there is something you have not read.
Choose What's new at the foot of the sidebar. On a narrow screen it is in the menu behind the button at the top. The same releases are listed publicly on the changelog, which you can read without signing in.
What it shows
Releases in date order, newest first and grouped by month. Each item is tagged as a New Feature, an Improvement, or a Fix, and the ones worth a paragraph get one. Smaller items are listed as a single line each. Where there is somewhere useful to go and look at the thing, the entry links to it.
About the dot
The dot means something has shipped since the last time you opened the page. It is measured from the later of your last visit and the day you signed up, so a new account does not start with a dot pointing at releases from before it existed.
The most recent releases are open by default, roughly a fortnight of them. Everything older sits behind Show earlier updates, which says how many it is hiding.