The encryption key for full disk or partial disk encryption are stored in RAM while the OS is operational.
How are the disk encryption keys protected when a laptop is locked?
If the device is shut down, then RAM is cleared, thereby securing your keys.
If RAM is really cold, it will last awhile longer, so one technique to extracting RAM is to freeze the device, and then quickly reboot to a specialized OS to capture such information.
If your device has been off long enough, especially at room temperature, then the RAM is completely inaccessible.
In stand-by mode the RAM is kept alive as long as power is available. Hibernate on the other hand qualifies as a true shut-down.
One of the commenters pointed that you one should check whether Swap partitions are encrypted. If they are not, then you have partially defeated the strength of your full disk encryption.
However, when your laptop is fully on, obviously all data is unencrypted and can be read by anyone.
If you have 'locked your screen', then it is not completely accessible. Even if the key is in RAM, it is very tricky to get the data out. One may also be able to attack your device via a USB interface, but this is tricky as well. There is the possibility you are running a vulnerable network service, so the WiFi or Ethernet interface is another possible attack vector.
What I would like to know is what happens when you lock the computer? ... if you lock your laptop, is your data in RAM still as safe as on your Hard drive when it is fully powered down.
No, the only way to get the true safety is a full shutdown or hibernate (again, make sure Swap is encrypted also)
Is the data in RAM protected somehow or is the data STILL readable but just behind a password?
RAM is not encrypted. ☺