when we reserve the time we get is the time we arrive to the park?

You should arrive about 30 or so before your time to account for parking going through the temperature check and security. So then when you get to the gate it should be your time to go in

I guess never heard of reserve the time.

No. Today I went and they did not check the reservation time. Our reservation was at 12:45 but we arrived around 11:30.