What is the problem in ARP that ARP POISINING happens?

Question

Hi, 

I was studying about Address Resolution Protocol(ARP) and I have two question :

 

1. what are the issues in this protocols that hackers can make problem in network as ARP POISONING ? 

2. and after find the problems I want to know how can I solve this problems?

 

please help me to know the issues and solves of these questions.

Thank you.