The Primary Vindex for a table is analogous to a database primary key.
Every sharded table must have one defined. A Primary Vindex must be unique: given an input value, it must produce a single keyspace ID. At the time of an insert to the table, the unique mapping produced by the Primary Vindex determines the target shard for the inserted row.
In Vitess, the choice of Vindex allows control of how a column value maps to a keyspace ID. In other words, a Primary Vindex in Vitess not only defines the Sharding Key, but also decides the Sharding Strategy.
Uniqueness for a Primary Vindex does not mean that the column has to be a primary key or unique key in the MySQL schema for the underlying shard. You can have multiple rows that map to the same keyspace ID. The Vindex uniqueness constraint only ensures that all rows for a keyspace ID end up in the same shard.