it means that someone is telling you something you want to hear and flatters you/plays with your heart strings, but has no real intentions of being with you or loving you.
like if you're dating this douchebag who cheats on you all the time and he tells you sweetly "baby, I love you. you're beautiful to me and I'll change for you...'' and blah, blah, blah, but he has no intention of changing and probably doesn't love you at all.
it's using romantic language to manipulate someone's emotions.
hope that made sense, haha
saying romantic but very shallow things. like nothing super deep and important just "oh honey you are so beautiful, I love your eyes blah blah blah..."
Whispering sweet nothings means that he's saying romantic things, "You are beautiful. I love the way you look. You make me happy" etc etc. It's flirting in the form of flattery.
lol well whenever I whisper sweet nothings into a girls ear its usually random; I mean I can say mushy stuff but I start with, "can I tell you a secret?" and just say something random EX. "taste the rainbow," or "sweetnothings" over and over or something in a foreign language. as long as I say it in a low calm soothing voice and brush my lips across her ear lobe they tend to like it.
hope I have helped