Trusted RFC cab be used to connect with other SAP without password.
To connect one SAP system with another SAP system without password, calling SAP system is known to the called system as a trusted system.
The calling SAP system must be registered with the called SAP system as a trusted system. The called system is called the trusting system.
Step by step procedure to create trusted RFC with screen shots:
Now we will establish trusted connection from ABC system to XYZ system
Here ABC is trusted system and XYZ is trusting system.
Now create normal RFC connection to ABC in XYZ system.