The main meanings of the word "και" are "and", "as well", "also", "plus", "at the same time" or it just indicates transition from one phrase to another.
Sometimes, though, it is used to intensify the meaning, or to make it milder, to indicate uncertainty. In those cases we do not usually translate it.
Δεν είναι πολύ καλό. = it is not very good. (absolute meaning)
Δεν είναι και πολύ καλό = It is not very good. (milder meaning. It includes a kind of uncertainty) "It might not be very good."