Presumably 'still' as in 'ongoing' - e.g. he is still sleeping/ mae e'n cysgu o hyd?
Yes, that is exactly it.
But, in this course we're using another meaning of this phrase and as a result the example above is deleted but still hanging around on some apps.
This is what is in the exercise:-
Hyd = length
O Hyd = 'of length' or 'long'
eg. Pedwar metr o hyd = four metres long