What does "read status" in admin area means? (General)
I was trying to translate forum to Traditional Chinese,
because the original "Chinese.lang" was outdated and has some uncorrect translate.
But I don't know what does "read status" meaning in admin area.
Is it means that user already read it or some kind of archive function (like read only)?
Let me try to explain it.
When a registered user reads an entry, the information about that fact (user X read entries Y and Z (and so forth)) get stored in a dedicated database table. When loading a new page of the forum, the script determines which entries the requesting user has read before. Based on that information the script adds elements and attributes (i.e. classes) to the HTML to mark links to read entries with CSS styling rules. This is the read status in itself.
When we reimplemented this feature , we had no clue, what amount of data this would generate. So we introduced three ways to limit the data that get stored for this feature. Beside the possibility to keep all data "for ever", we reintroduced the old way to limit the data usage (user based limit of information for number X of entries) and two new ways (both time based: remove information after X days or when automatically closing the threads after X days). That willl beregulated in the first setting block at top of your screenshot.
Second part of settings is the bottom setting block in your screenshot. If one selected to store the read status only for a number of days or entries, this setting declares the concrete value of days or entries to store informations about. If one keeps the default value of 500, the script stores the information, that a specific user read a specific entry for 500 days  or it throws the information about the oldest stored entry away if the database table keeps information about 500 entries and the user reads her/his "next" entry so the limit would overflow.
For keeping all data "for ever"  we need no further settings and the function to remove the read status if the thread gets automatically closed, is an own, independent setting of the forum.
I hope, my explanation gave you the needed clearness. If not, ask for more details.
: Before the feature was cookie based and worked only inside one browser. If I read an entry on my phone, it was not marked as read on my notebook or my desktop machine at work. Now it is user-account based and I see on machine A what I read on machine B or C.
: The experience from this (not well frequented) forum shows, that the data amount is far from overwhelming. Here the data about who read which entries are kept for ever. Within more than three years since the introduction of the feature, the forum stored over 10000 datasets for entries, that was read by several registered users. The backup function generated a file with a size of only 1 Megabyte (with the overhead of SQL instructions that makes more than 50% of the file size).
Trenne niemals Müll, denn er hat nur eine Silbe!