Essentially melody line that isn't repetitive and isn't abrupt.
To achieve this we repeat certain aspects of the melodies but vary other aspect. So the melody line is similar to what was sung before to feel the connection but different enough that we see it as a different melodic line. If you want to be technical using motivic development when writing melodies.https://gottrypercussion.com/wp-content/uploads/2016/07/Theory_MotivicDevelopmentHandoutandWorksheet.pdf
Gold standard of any melody line would be Mozart Eine kleine Nacht Musik. Almost every single melody line is the exposition is different to each other.
Verdi famous aria La Donna Mobile is another great example of a melody with little straight repetition.
In pop/rock music
Genesis - Dancing in moon lit knight
Kinks - Some mother's son
Beach Boys - Don't Talk
Beatles - Yesterday
Beatles - I Should have Known Better (the chorus in particular)
Sparks - Slow boat
With pop music, there will be inevitably be more repetition compared to classical themes. However with top class melodic writing there will still be a continuous flow of different melody lines for about 4 lines before repeating itself. Or if they do repeat a line, they only do it once before moving on. Listening to one line being repeated mulitple time is a lot more tiresome than listening to a block of 3-4 lines of melody being repeated or one line being repeated once and moving on.
In fact it was a rule I had when I was writing music. Never repeat a phrase of music more than once without alteration. i made sure I tried to stick with that.