Answering the first part of that question:
An overflow protection means either an addition to a storage-related circuit or a modification to it, which makes that circuit not misbehave when you either dump too much into it at once or put in too much in total.
For example this is the basic classic item sorter:
And this is the overflow protected version of it:
The smaller version only needs one of the to-be-sorted type inside the filter, the bigger one needs 41. But if you put so many of the item into the storage that all the chests and the hoppers leading to them fill up, then the small one not only goes from power level 1 to power level 2, but to power level 3, causing a power level 1 in the second piece of redstone wire of adjacent filters, therefore causing them to push all their filter items into their storage, which causes all further items to not get sorted at all, but get directly dumped into the storage there instead. That quickly leads to the signal of those going to 3, which breaks another filter next to it and so on, it cascades to a big mess to clean up manually.
The bigger version however is at power level 2 always and goes to level 3 when an item goes in. But even if items try to go in at infinite speed or when all the chests and hoppers fill up, you still can't get more than 64 of that item in the filter hopper, therefore the power never goes above 3 and it doesn't break the other sorters. Further items simply don't go into the filter and instead continue in the upper hopper line. At the end of it you can build an overflow storage for all items that can't get sorted (like pickaxes), items that don't have a filter (like diorite renamed to "fdjslkfsdv") and items that overflow their storage. And even if that fills up completely and all the hoppers in the upper hopper line fill up, nothing breaks permanently, it just continues sorting when you make space for it.
This concept is not limited to sorters, item transport systems can also have overflow protection, farms can shut down as an overflow protection and so on.