I'm pretty sure it means there was grease or something else on the die itself at the time of striking, which caused the mint mark not to come out clear, hence the space in between the "S" is filled
I have found (4) 1979s halves and they all look the like the filled s variety. If you have some other proofs take a look at the s and you will see that it is much more clear and easy to read compared to the filled s variety.