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 bookmarks on apps, click profile picture to go to your profile page, then click 🔖 or "Bookmarks".

To view bookmarks on web, click 🔖 or "Bookmarks" on right of screen.

Bookmarks vs Favourites explained: https://fedi.tips/how-to-use-mastodon-and-the-fediverse-basic-tips/#FavouritesVsBookmarksInMastodon