Extra Magic Hours are offered each week, once in the morning and once in the evening, for each park (that is, every week each park will have morning EMHs once and evening EMHs once). Morning EMHs means the park opens to resort guests 1 hour early (usually that means 8 am instead of 9); evening EMHs means the park stays open 3 hours late (the hours vary by park and by season; at AK it's usually 5pm-8pm or 6pm-9pm, but at MK it can be as early as 7pm-10pm or as late as 1am-4am). It's the second-best perk to staying on property, in my opinion (after Magical Express).
However, what people are talking about on this thread is different. This thread is about simply making a reservation at the restaurant for before the park is open--the idea is that you get in early and go eat breakfast, and then by the time you eat breakfast, the park *will* be open. Anyone can do it but you can't go into areas of the park that are not open yet.