Available is the immediatley spendable amount of bitcoin stored in this wallet. It is the Total less Confirming and Locked balances. If you have enabled Use Unconfirmed Funds setting then the Available amount is the Total less Locked balance.
Locked is the amount of bitcoin stored in this wallet that is allocated as inputs to pending transaction proposals. This amount is determined using unspent transaction outputs associated with the wallet and may be more than the actual amounts associated with your pending transaction proposals.