Explanation
A buffer overflow (or buffer overrun) occurs when the volume of data exceeds the storage capacity of the memory buffer. As a result, the program attempting to write the data to the buffer overwrites adjacent memory locations.
Buffer overflow is a vulnerability in low level codes of C and C++. An attacker can cause the program to crash, make data corrupt, steal some private information or run his/her own code. It basically means to access any buffer outside of it's alloted memory space. This happens quite frequently in the case of arrays.

Explanation
https://www.cisco.com/c/en/us/products/security/what-is-a-ddos-attack.html

Reference:

https://www.cisco.com/c/en/us/support/docs/security/email-security-appliance/118796-technote-esa-00.html