Being transparent means being 100% completely honest with your partner. This actually sounds a lot like common sense: if you want a healthy and happy relationship, you need to be honest. Obviously.
Though this is pretty much common sense, so many individuals have trouble being completely transparent with their partner. Maybe they feel that if they're honest all of the time they will create tension, cause arguments, open up some emotional wounds, etc. so they would rather avoid being transparent and not deal with any of those issues. Problem is, however, a relationship really can't succeed if there is no transparency.
Complete transparency is a sign of maturity, love, and respect. When you're the one who needs to be honest, you do it because you love and respect your partner. You may need to talk to them about a rude comment they said or a poor decision they made, and you tell them because you don't want them to say or do those negative things again. When you're the one receiving the transparency, it's a sign of maturity that you accept what is being told and that you will put forth effort into making necessary changes.
Some topics may be difficult to communicate, and they probably are harder to hear, but having that openness helps you to strengthen your bond and overall positively affects that maturity, love, and respect you have for reach other.