Destination in the United States at time of admission is the field 3.a on Form I-90. Form I-90 has two special fields that are related to the place where you entered the United States and where you were going in the U.S. The destination at time of admission is simply the place where you headed for.
For instance, you may have arrived to the United States in Philadelphia, Pennsylvania, but you were actually moving to Pittsburgh in the same state. In this case, Pittsburgh is the destination in the United States at time of admission whereas Philadelphia airport is the point of entry in the U.S.
As Form I-90 indicates, you should enter data in these fields only if you arrived to the U.S. with an immigrant visa. If you applied for a green card while already being located in the U.S., do not fill out the items 3.a and 3.a.1 in Part 3 on Form I-90.