Hello,

PDU is stands for Protocol Data Unit. It is a term used in OSI model , which refers to a group of information added or removed by a layer of the OSI model or form of data at every  layer of OSI model.

PDU is a significant term related to the initial four layers of the OSI model. In Layer 1, PDU is a bit, in Layer 2 it is a frame, in Layer 3 it is a packet and in Layer 4 it is a segment. In case of layer 5,6 and 7 PDU is referred to as data.

For example:- As we know that RIP is  a layer 7 protocol . so whenever a router generates RIP update at layer 7 called Data till layer 5. Then it will handover the Update message to layer 4. Now RIP uses UDP as a transport protocol. So Transport layer(layer 4) adds UDP header. After adding the UDP header , now its called Segments. Then Segments received at layer 3 and layer 3 will add IP header. After adding IP header is known as Packet and further received on layer 2 and known as Frame.

Hope it helps.

Thanks