Hi there!
There are different kind of bullet trains, classified as ''Nozomi'' ''Hikari'' and '''Kodama''. The difference between these trains is the amount of stations they stop at. Where the Nozomi makes the fewest stops, the Kodama makes the most. The above statement simply means that you can't board Nozomi trains but you can board all other kind of trains. As a JR Pass user you won't have to worry about this, when you want to make a booking, simply show your JR Pass to the JR Staff and tell them where you wish to go. They will print the right tickets for you.
The same appies for the Mizuho trains, these are basically Nozomi trains in the south of Japan.
As for Reserved / non-reserved seats. Each train comes with a limited about of reserved seats and non-reserved seats. You can make a seat reservation if you wish to be sure of a seat and don't want to line up before hand to get a seat. Seat reservations are free with the JR Pass, so I would recommend making them to make your travel more relaxed.
I hope this clears things up!
Daniel