The goal of a walking bass-line is to outline the chord progression but with an interesting melodic line that provides a counter-melody (as above).
And walking bass-lines are a continuous sequence of quarter notes, generally played on the beat (4 notes per bar in 4/4 time). A bass-line is a melody line played in the bass (by a low-pitched instrument such as the piano, double bass, electric bass, tuba, etc.).
