Voting is the mechanism that keeps a server list ranked. Every vote counts towards a server's position, which is why so many servers ask their players to vote daily.
How a vote works
You open a server's page on the list, confirm you are not a bot, and submit your vote. The vote is recorded against that server and counted towards its ranking. You can usually vote once per day, per server.
Why servers care
A higher position means more visibility, and more visibility means new players. For a server owner, votes are one of the few free ways to grow a community.
What you get out of it
Many servers reward voters automatically with in-game currency, crate keys or items. If a server supports this, the reward is delivered to your account shortly after you vote - just make sure you enter your exact in-game username.