Dora Bas Rivka Silver O'H
"Shouldn't Talk About It on Shabbos"
G-d tells Isaiah to call out to the people of his generation, like a shofar, informing them of their sins. They seek G-d with their words, but not with their deeds. They ask to know G-d's will, but they do not intend to fulfill it. The people complain that they pray and fast, yet G-d does not answer their prayers. This is because, while they go through the motions of fasting, they are also pursuing their business and doing their regular daily transactions, collecting debts. They fast leading to arguments and trouble - how can G-d accept such a fast? He can see what's in people's hearts. He won't accept fasting or other forms of repentance unless they're accompanied by the proper motivations. One has to give up their bad ways and act charitably to those less fortunate. Feed the hungry, clothe the naked - don't pretend you can't see them!
If you actually change your ways, then you will be prosperous and successful. G-d will heal you and your merits will go ahead of you into the Next World. If you turn away from evil, G-d will answer your prayers. If you are charitable to those in need, your times of darkness will be as bright as noon. G-d will always lead you and you will never lack, like a well-watered garden. Just as your bad deeds led to destruction, your good deeds will cause the world to be built up for generations to come.
If you refrain from doing your business on Shabbos, in word as well as in deed, then you will rejoice with G-d, Who will reward you.
These last few verses are the source of not running on Shabbos (except to shul or a mitzva), not doing "un-Shabbosdik" things (like preparing on Shabbos for after Shabbos) and "shouldn't talk about it on Shabbos." (These principles are respectively known as hiluchecha, cheftzecha and dabeir davar). Halachos derived from Navi are even more authoritative than Rabbinic laws and some say they're as authoritative as actual Torah laws. This synopsis is not the place to go into these halachos with any depth. It's just something to think about the next time you hear someone say, "Shouldn't talk about it on Shabbos!"