Before joining Minecraft Whitelist Servers you have to be approved and added to Whitelist. On some Whitelist servers you have to contact server Owner to be added while on others you can simply visit server's website and fill out application.
Why are some servers using Whitelist?
Most of servers use Whitelist simply to ensure only high quality players join, they want to stop people who just want to raid or annoy other players.
Since only the players on Whitelist can join those servers are usually well organized and don't let griefers in, also be sure to check out rules as they're generally very strict.