Buffer-Overflow and memory slots-pointers

I'm currently studying coding errors in C and Java that may lead to a buffer overflow.

I understand conceptually the buffer overflow. My big problem is that I don't understand all the "memory process": for example "0xFF" I don't see where this comes from.

**Anyone can shed some light and recommend a succinct source to understand how the memory and its slots work? so that I fully comprehend buffer overflow**

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