Blockstream Green 2FA Protected accounts offer an additional layer of protection for your funds, by requiring a one-time code from your email, phone, or authenticator app in order to spend from your wallet. This is possible because 2FA Protected accounts are actually 2-of-2 wallets, with one key needed to spend funds held by the user (in the form of the recovery phrase) and the other key held by Blockstream. When the correct 2FA code is entered, Blockstream will sign the transaction with its key.
In order to avoid blocking access to user funds if Blockstream is unable to sign using its key, 2FA Protected accounts transition to 1-of-2 wallet after a specified timelock period (default ~1 year), at which point funds can be spent with only the user's recovery phrase. This means that Blockstream is no longer needed to sign transactions, but also if an attacker finds your recovery phrase, they do not need your 2FA code to steal funds.
The ~1 year timelock period on your funds begins each time a coin is received or sent back to your wallet as change. If you are frequently sending funds to and from your wallet, is it likely that the 1 year timelock period will refresh naturally. However if you do not send many transactions and want 2FA protection on your funds, you will need to manually re-enable the timelock after some time by sending (or redepositing) funds back to your wallet.