Before a transaction is submitted to the Stellar network, it needs to be signed by one or more signers. To sign a transaction
When no custom signer is required
This is the most common case, all you just need to do is enter your password, and Saza will sign the transaction with the appropriate keys from your wallet.
When custom signer is required
Saza will automatically inspect a transaction and let you know if a custom signer is required. In this case, you need to enter the secret key for the custom signer in order for Saza to be able to sign and submit your transaction