The iPhone/iPad version of the official Mastodon app had an update last week which includes the Bookmarks feature. These let you save interesting posts to read later.
To bookmark a post on apps or web, click ⋯ and then click "Bookmark" (or just ? ).
To view your bookmarks on the apps, click your profile picture to go to your profile page and then click ? or "Bookmarks" (depending on the app).
To view your bookmarks on the web interface, click on ? or "Bookmarks" on the right of the screen.