If you enable full disk encryption during the installation, you need to remember the password. Since you will need the password before you able to see the OS login page. It seems like a harddisk password in your BIOS to unlock your harddisk before login to the OS.
Remark: You need to remember your password, otherwise you will lost all of your data. And also, you need to backup your data regularly.
Unless your machine is powerful on CPU and also using high speed SSD, otherwise, you will see the performance impact on the encryption. It is not recommend to enable on the old machine.
