NOI is notification of interest from a province like Ontario. If it's from Ontario you will have 45 days to accept it, means you have to submit all the docs and pay fee in 45 days(check the detailed procedure in thier website, they given it pretty clearly). If you got nomination from them(means Ontario approved your application), you will get 600 points in your EE profile and will pretty much end u in an ITA in the next draw.
Now coming to eligibility, they are selecting people in batches depending on CRS scores from this yr onwards. You can check the updates page in ontarioimmigration to know more on the trend.