The question we are looking at in this episode is: There is a sale I want to go to right after shabbat? Can I walk there on Shabbat so I can be there right as Shabbat ends?

The prophet's words are a form of "rest" to protect spirit of Shabbos.

As its the prophet's speaking it carries weight.

In Yeshaya it says "you shall honour the Shabbos in your goings by refraining from pursuing your weekday occupations and from speaking of them".

Basically referring to 3 elements:

1) Walking

2) Weekday occupations

3) Speech