Indicates how many Bytes are used for the actual signal, and how many for correction of errors.
A FEC of 1/2 means that 1 Byte out of 2 is used for error correction, while a ratio of 7/8 means 7 Bytes are used for the actual signal, and only one for error correction.

A FEC of 1/2 gives as perfect as possible reception, since every Byte containing actual signal is controlled by another Byte checking it.

When a provider chooses a FEC of 7/8 it means he is not wasting any bandwidth at the cost of delivering a signal.
The lower amount of error correction means that more sophisticated equipment at the receiving end (for example a more stable and sensitive LNB, or higher reserves with the dish) are needed compared to the same transmission using a FEC of 1/2.