1.what is mean by message in bulk in basicNack
The AMQP 0-9-1 specification defines the basic.reject method that allows clients to reject individual, delivered messages, instructing the broker to either discard them or requeue them. Unfortunately, basic.reject provides no support for negatively acknowledging messages in bulk.
2.is basicNack with multiple false is same as basicReject?
channel.basicNack(deliveryTag,false,true);
channel.basicReject(deliveryTag,true);
Yes; it is effectively the same.