Master and Masterless refer to the way your accounts (keychains) can be restored.
The keychains in the Masterless section can only be restored by importing the private key of each of these keychains, one by one.
The keychains in the Master section can be restored all at once by entering the 12 words that make up the keyphrase.
Keychains can not be moved from Masterless to Master. To add keychains from the Masterless section to the Master section, funds need to be send to a keychain already present (or newly created) in the Master section.
ALWAYS save your keyphrase and private keys securely. You lose access to your funds without them.