Ack messages are used in TFTP transfers. Every packet a sender (IPO) sends will be ACKnowledged by the receiver (Manager) who sends a ACK message with the sequence number of the packet he just received to the sender.
If the receiver don't receives the next packet within a predefined timeout he will re ACK the last packet received so the sender knows he need to resend the packet.
So in case of the manager this means he missed a packet from the IPO and resends the last ACK. This can be due to various reasons (corruption, collision, long delays, firewalls,...).