C++ is a systems language. It doesn't mandate any checking on operations: a program which is implemented correctly would be impeded by unnecessary checks, e.g., to verify that an array access is within bounds. The checks would consist of a conditional potentially needed on every access (unless the optimizer can see that some checks can be elided) and potentially also extra memory as the system would need to represent the size of the array for the checks.
As there are no checks, you may end up manipulating bytes outside the memory allocated for the array in question. That may override values used for something else or it may access memory beyond a page boundary. In the latter case the system may be set up to cause segmentation fault. In the former case it may access otherwise unused memory and there may be no ill effect. ... or it may modify the stack in a way preventing the program from correctly returning from a function. However, how the stack is laid out depends on many aspects like the CPU used, the calling conventions on the given system, some compiler preferences. To avoid impeding correct programs there is no mandated behavior when illegal operations are performed. Instead the behavior the program become undefined and anything is OK to happen.
Having behavior undefined is kind of bad if you cause it. The simple solution is not to cause undefined behavior (yes, I know, that is easier said than done). However, leaving behavior undefined is often good performance and it also enables different implementation to actually define the behavior to do something helpful. For example, it makes it legal for implementations to check for undefined behavior and report these cases. Doing something to detect undefined behavior is, e.g., done by -fsanitize=undefined provided by some compilers (I don't think that would cause all kinds of undefined behavior to be detected but it certainly detects some kinds of undefined behavior).
