TCP segment of a reassembled PDU
Wireshark Q&A https://osqa-ask.wireshark.org/questions/58186/tcp-segment-of-a-reassembled-pdu/
It means that
- Wireshark/TShark thinks it knows what protocol is running atop TCP in that TCP segment;
- that TCP segment doesn't contain all of a "protocol data unit" (PDU) for that higher-level protocol, i.e. a packet or protocol message for that higher-level protocol, and doesn't contain the last part of that PDU, so it's trying to reassemble the multiple TCP segments containing that higher-level PDU.
For example, an HTTP response with a lot of data in it won't fit in a single TCP segment on most networks, so it'll be split over multiple TCP segments; all but the last TCP segment will be marked as "TCP segment of a reassembled PDU".