Redundancy means sending extra bits along with the actual data — bits that are mathematically derived from the data itself (e.g. parity bits, checksum, CRC remainder) but carry no new information on their own.

How it enables error detection:

1. The sender computes these redundant bits from the original data using an agreed-upon algorithm (e.g. XOR-based parity, polynomial division for CRC) and appends them to the message before transmission.
2. The receiver, upon receiving the message, independently recomputes the same redundant bits from the received data using the identical algorithm.
3. It then compares its freshly computed redundant bits against the redundant bits that were actually received (sent by the sender).
4. If they match, the data is assumed to have arrived without errors (the check is consistent). If they don't match, it means the data was altered in transit — an error is detected.

The key idea: since the redundant bits are a deterministic function of the correct data, any change to the data bits during transmission (that isn't precisely compensated for) will very likely cause a mismatch, revealing the presence of an error without needing to know the original data independently.