Ugh, really sorry I only noticed this thread now. I really should do some kind of FAQ / Help section of Pinside features
Okay, about Bookmarks. There are two types of bookmarks on Pinside: favorite threads and bookmarked posts.
Favorite threads can be marked using the star icon at the top and bottom of any thread you visit. Once you favorite a thread, it will show up in your "favorite topics" here: https://pinside.com/pinball/forum/topics/user-favorited
Hit this button to fav the topic!
Bookmarking specific posts can be done using the little bookmark icon in the bottom left under any post. A pink coloured window pops up and you should now choose to save an actual bookmark in your bookmarked posts, or simply save your read position in a thread (e.g. like an old-fashioned bookmarker).
When you store a post in your bookmarked posts, you can add a summary of your own and even tags (which will work as a filter - but that's not implemented yet).
Here I create an entry in my bookmarked posts
And then you will find all your bookmarked posts here: https://pinside.com/pinball/forum/posts/user-bookmarked :
There it is!
When you only store your read position, this will only display an icon for that thread and when you click it you will be returned to that post in the thread.
This thread contains a bookmarked post, click the yellow box to jump to it and carry on reading from there!