A keyspace is a logical database. If you're using sharding, a keyspace maps to multiple MySQL databases; if you're not using sharding, a keyspace maps directly to a MySQL database name. In either case, a keyspace appears as a single database from the standpoint of the application.
Reading data from a keyspace is just like reading from a MySQL database. However, depending on the consistency requirements of the read operation, Vitess might fetch the data from a primary database or from a replica. By routing each query to the appropriate database, Vitess allows your code to be structured as if it were reading from a single MySQL database.