/System/Library/PrivateFrameworks/IDS.framework/identityservicesd.app/Contents/MacOS/identityservicesd identityservicesd is a daemon in Mac that handles user account credentials on the device like iCloud, iMessage, App Store, FaceTime, etc. This allows user to automatically sign in to various accounts without needing to enter the account credentials. But, this process requires Keychain password to access the passwords stored in Keychain. If it doesn't get the keychain password, it will ask for it with the identityservicesd wants to use the login keychain prompt.
The binary is a part of identityservicesd application.