If you are accepting into a PNP program and are nominated you will be awarded extra points towards your express entry application. The provinces can also withdraw a nomination for various reasons which would mean you would have to decline the invite.
Yes, the provincial restriction is printed on your PR card and you have to work and live in that province for a certain time (2 years). The downside is that the province might not have good jobs or salary for your line of work in which case you will have a hard time.
If you accept the nomination, the system will find that you meet the requirements of the PNP, and you will get a letter in your account that confirms your nomination. Your profile will be placed into the Express Entry pool and you will be awarded 600 additional points that will help you get invited to apply. (Note: 600 points is the maximum additional points you can get. If you have points for a job offer and/or study in Canada, only the 600 points for your nomination will be added to your CRS score.)