I think "openness" has two parts: first, honesty, and second, to trust the other person with everything and anything. These two things cause a renewal in the relationship/friendship, to let both people know that the feelings and convictions previously felt are still true. Giving the other person some of your time and attention implies you still care about him/her even if you can't give the other person all of your time, which would be unreasonable anyway. Of course, this doesn't mean you have to make calls routinely daily, but just a simple thing to let the other person know your feelings and convictions haven't changed.
Ideally, a couple should be open with everything and able to speak of anything and everything. This does not mean that each person needs to be constantly talking to each other, but because openness is good for building trust, it will help you become more comfortable with the other person. Especially important topics should be open to the other person, so as not to let them be deceived.