Final answer:
The signal aspect 21-132 typically refers to a green signal, which indicates permission for the train operator to proceed.
Step-by-step explanation:
In the context of signal aspects in trains, the signal aspect represented by the code 21-132 typically refers to a green signal. A green signal indicates that the train operator has permission to proceed, indicating a clear track ahead. This signal is used in railroad signaling systems to maintain safe and efficient train operations.