SASL stands for Simple Authentication and Security Layer, and it is a standard (not just for IRC!) way of securely authenticating your identity to a server.
AfterNET servers support SASL login, and is the recommended way of connecting.
There are different types of SASL available, here are the settings you should use to connect to AfterNET:
Once configured, connecting this way has the benefit of bypassing extra connection-time security checks, and ensures your identity is configured before you join any channels.