Briefly, the difference between the option "POP" and "IMAP" is how you choose to read and download your e-mails.

When selecting "POP", your account is downloading your emails from inbox to the mail client you are using and those mails will then not be saved on our servers (webmail). You will therefore not be able to read your email from multiple locations and clients, instead the emails are stored locally on the client where you last downloaded your e-mails to. Many mail clients offer an option to save a copy of the email on the server so you can read your mail from multiple locations. With POP settings, the other folders (e.g., Sent) are not synchronized with the server and will only be displayed locally in the client you have sent the mail from.

If you instead choose "IMAP", your emails will be saved on our servers so that you can read, write and send emails from multiple locations or clients.
Synchronization applies to all folders, which means that your email looks the same in all connected clients in different devices (computer, phone and / or tablet).