Okay kiddo, do you know what a bass is? It's a big guitar that makes low sounds. Now, a fretless bass is like a regular bass, but it doesn't have metal pieces on the neck called frets that help players know where to put their fingers for each note.
Instead, a fretless bass neck is smooth, like a slide at the playground. So the player has to use their ears and memory to know where to put their fingers to make the right notes.
It's like playing hopscotch without the squares or playing tag without a base. It can be a little tricky at first, but with practice and patience, a musician can become very good at playing a fretless bass and make some beautiful music.