https://www.musical-u.com/learn/what-gives-the-fretless-bass-its-sound/
The fretless bass has a very distinctive sound.
Whether it’s a fretless electric bass or a traditional upright double bass, there’s something about its sound that marks it out clearly from a normal bass guitar. What is it?
There is one big factor: the lack of frets!
To understand the significance of this you must understand why bass guitars have frets at all.
Read more in this interesting article from Musical-U.