Actually, it means nothing.
Valid means ‘true’ or ‘real’. The term licity (legal) or illicit (not legal) is used by Latin Catholics to indicate that the Orthodox do their sacraments without the permission of the Supreme Pontiff at Rome.
However, Eastern Orthodox Catholics have never ever needed the permission of the bishop of Rome from Pentecost to the present day, since he never had such authority outside of his own Metropolitan See.
So what we have here is a legal fiction.
However, according to the opinion of the Roman Catholic church Orthodox Holy Mysteries (sacraments) are indeed valid, regardless. We are, of course, flattered :tiphat: . (But confidentially, we did not ask and did not need the Roman Catholic church to tell us this, we already knew

)
This is interesting, of course, because both churches affirm that the Holy Eucharist is a sacrifice, but it is not a reenacted event, it is one eternal cosmic sacrifice outside of time, which we ‘inside’ time can participate in.***
So then, if the Orthodox liturgy
is*** valid (regardless of permissions

) and the Roman Catholic liturgy is also valid, we are participating in the same cosmic eternal timeless event together, and in heaven we are already in communion before Christ God.
At least, that would be [theoretically] how some would see it.