If you have to ask it already means absolutely nothing to you.
Memory bandwidth is the maximum theoretical speed at which memory(RAM) can be read. So the theoretical speed at which a CPU or GPU can retrieve data from memory. It's important to keep the processor fed with data, because if it is too slow the whole process will stall because the processor processes data faster than it can retrieve it and has to wait for new data. In games that would result in stutter or lower framerates. More bandwidth is better but it is NOT a direct indicator of overall performance, since a game can also be bottlenecked at the processor, which is usually the case.
I/O throughput is similar. It refers to a specific storage device, which could be RAM or non-volatile storage like an HDD or SSD. Again, also not a direct indicator of performance but higher throughput is always better. As far as I have read it is more used for non-volatile storage. It's more important for next gen because the GPU will be able to directly access the SSD for additional data. However SSDs are much much slower than RAM, so the SSD needs to be as fast as possible to be of use to the GPU.
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