On a Mac, your keychain password is generally the password you use to log into your Mac user account. The keychain is where web site and other passwords are stored. Turbotax uses the Safari webkit (a web browser component) to access and download the ItsDeductible web site. The Mac system is asking you to provide your login password to authenticate that Turbotax is allowed to access your stored passwords so it can log into ItsDeductible.
(This prevents programs that you might not want to access your passwords from doing so without your permission, such as if you accidentally install malware or a virus that wants your passwords.)
