WP Login Delay is a plugin that adds a one second delay when logging into the system in order to slow down any brute-force attack on your website.
Just think about it... If you want to pass through a huge door and you have to try thousands of different keys to find the correct one. You will test all of them until the correct one works, right? This means that for each attempt, you will have to wait for the result to know if it is the correct key or not. Each result being delayed, it will take a lot more time to find out which key is the correct one. It is the same principle that is used here, one second delay is ok for a human, but looks like eternity for a bot.