The analogies in the post seem a bit overly complicated to me. Keep it simple.
Let's say I have an empty glass (memory allocation). I know ahead of time that the maximum capacity of that glass is 8 ounces. A buffer overflow is what happens when I overfill that glass. If the amount of water in the glass stays less than 8 ounces, no problem. If it overfills, I don't quite know where it'll go. It might get into something it shouldn't be in (security, program instability, etc).
Let's say I have an empty glass (memory allocation). I know ahead of time that the maximum capacity of that glass is 8 ounces. A buffer overflow is what happens when I overfill that glass. If the amount of water in the glass stays less than 8 ounces, no problem. If it overfills, I don't quite know where it'll go. It might get into something it shouldn't be in (security, program instability, etc).