How does DROP rule work in iptables?

I'm working with iptables in order to close one or two open ports.

So I tried this rule in order to change the default policy.

# iptables -A INPUT -p tcp --dport 3333 -j DROP

Then I checked the policy using iptables -L -n -v where it showed the port which was dropped.

I don't have a basic understanding on how does the DROP rule works? Any help would be appreciated.