I/O throughput is howfast you move (all) data around the entire system. A cool new feature, of these SSD's is they are so fast, they dont have to store as much in Ram, as previous consoles did. You can now load things as needed, and the speed is so fast, its no longer needed to "pre-store" as much. You can even flush things out of system ram, as they go out of view, and load them as your character is turning around ingame, back into ram, because the speed is so fast, without it being noticeable to the player.
This means ram sizes (while just 16GB) actually funktions way above what you would expect, comapired to older consoles. Naturally to take advantage of this, you need a fast SSD and a high I/O throughput (or it would stutter, when you try this).
Memory Bandwidth, is just for your systems ram (in this case).
Its usually used in connection with video cards (because this is where memory bandwidth matters most, for gamers).
Higher resolutions need high amounts of Vram, but also higher memory bandwidth. Picture sizes (4k) requires alot more data moved around, and memory bandwidth matters for this. Basically Memory Bandwidth is required upto a certain point, and the lower resolutions the less the need for high amounts of it, and vice versa.
Look at the Geforce 3070: 448 GB/S memory bandwidth
Nvidia only gave it 448 GB/s, but thought that was enough for it (with its capabilities). Its about not running into bottlenecks.
How close are the PS5/XSX to this (in terms of performance)? *shrugs*
However it shows the consoles both probably have good enough memory bandwidth for it not to be a issue.
Xbox Series X has "weird" to use memory bandwidth.
Its system ram runs at 2 speeds (10GB at 560GB/s and 6GB at 336GB/s).
If a game needs more than 10 GB of ram to run, the mix-match of the speeds, might result in the PS5 memory bandwidth overall being higher.
Playstation 5's ram all run at the same speed (448GB/s) (so you dont need to worry about, data access levels/requirements, how much Vram games take up ect). It just all works, if you load something into it, while on the XSX the speed differnces might result in issues (depending on how its handled).