Oplop makes it easy to create unique passwords for every account you have. By using some math, Oplop only requires of you to remember an account name and a master password to create a very safe and secure password just for you. You get to choose the account names you use for each of your accounts so they act as a mnemonic, letting you make sure they are memorable. And your master password you only have to choose once as you use it for every account name you have (hence the "master" part). That means you can have safe and secure passwords for all of your accounts simply by remembering one master password and easy-to-remember account names for each of your accounts you use Oplop with.
For example, let's say I have an account at Amazon and another one at Google. First I need to choose account names for these two accounts. An account name can be anything I want it to be, just as long as I can remember it. For Amazon I will choose the account name Amazon and for Google I will choose FizzBuzz. What you choose for an account name is entirely up to you; it is merely a mnemonic you have for the account so that you can easily recall it any time you need the password for that account. In technical terms the account name acts as a nonce.
Next I need to choose a master password that I will use with all of my account names. Let's say I choose the password secret password. If I go to Oplop and enter in Amazon as my account name and secret password I will get a unique account password to use at Amazon: sar4_zIs. If I enter FizzBuzz as my account name and secret password as my master password (remember, you use the same master password for all of your accounts), I get yyexMS5c as my unique account password for Google.
Every time I use Oplop and enter the same information -- such as Amazon and secret password for my account name and master password, respectively -- I will always get back the same unique account password of sar4_zIs. The same regularity applies to my Google account.
And notice how random-looking those unique account passwords are? That's the math at work on your account name and master password. Making the unique account passwords seem so random prevents people from guessing your password for each of your accounts. And even if someone manages to get your password for one account, you are still safe since Oplop makes a unique password for every account name you use. Oplop also makes sure there is at least one digit and one letter in every unique account password in case a web site requires that sort of thing.
Thanks to all of this, the only thing you have to really keep secret is your master password. You can write your account names on a piece of paper you keep safely somewhere as long as you never tell anyone or write down your master password and make sure it is strong (i.e. complicated and could not be guessed by anyone who knows you).