It's simple it says what it does you arrive in Thailand get 60 days on arrival then if you want can extend for d further 39 days so in total you get 90 days that's it simple
If you fly back in to Thailand you will get another 60 days but don't do this to often you could be denied entry. There is no rest restriction but it depends on the airport immigration.
This visa sounds good if your not 50 years old then you can get retired visa etc ..it's a good way to stay in Thailand ..still expensive I think but if it works ok