To become a permanent resident you have to have a US citizen who is going to sponsor you financially. Normally people are adjusting their status to PR inside the US and their spouse (immediate relative sponsor) is also serving as their financial sponsor. If the spouse can't serve as a financial sponsor you can get a joint sponsor. I had to get my sister-in-law to sponsor me because my wife decided to go to law school and not work.
In this situation it appears your daughter and son-in-law are wanting to move to the US. As such, your son-in-law needs to financially provide for your daughter. He can only do that if he has a job in the US. He cant use his canadian job because they aren't going to be living in canada. I believe that is why the letter states he needs a job offer.
There are other ways to meet the financial requirement though. You can do it with assets or savings. Also, you can get a joint sponsor. If his parents, sister etc are willing, they could probably sponsor your daughter. See form I-864 on the USCIS website, that'll explain what you have to do.
