If refers to staying in Canada 2 out of every 5 years in order to meet the residency obligations and keep your PR status.
They do not have to be consecutive years. It is enough to stay 730 days in small increments as well.
You must have these 730 days in Canada in your first 5 years as a PR and after you have been a PR for more than 5 years, you always need 730 days in the last 5 years.
You can see this on the CIC website for example at http://www.cic.gc.ca/english/information/applications/guides/5445ETOC.asp#appendixA