Save posts
Keep a post past every rule that would remove it, and find it again on Saved.
Save a post
Press Save on any post, in any list, or press s on the post you are on. Press Remove from saved to stop keeping it.
Find what you saved
Open Saved in the sidebar. It lists every post you kept, and it is the one list nothing prunes. On Paid and Premium you can also label saved posts and read one label at a time.
What a save keeps
A saved post stays when its feed's span runs out, when you unfollow the feed, and when the feed stops listing it.
How many you can save
Free keeps 100 saved posts; Paid and Premium keep more (see Plans and limits). When Saved is full, a new save says so: remove one from Saved to make room, or move up a plan.
Questions
Can I take my saved posts with me?
Yes. Export your data includes every saved post's title, link, excerpt and labels.
Does a saved post count against the posts my plan keeps?
It counts, and it is never the post Reader removes to make room. When you reach your plan's posts, Reader takes back posts you have read and not saved, oldest first. See I'm over my plan's limits.