Both the uniqueness constraints and primary keys constraints ensure that each row for the specified column or set of columns is unique. There can be one primary key per table, but multiple uniqueness constraints.
Each new table has an inbuilt primary key named "_Record_ID".
On the other hand, uniqueness constraints can be specified by the designer.
Select the columns used for indexing and then press "Apply":
Note that columns used as primary keys, indexes and uniqueness constraints can't be deleted.
Unique Constraints and Unique Indexes", for tips regarding designing proper constraints.