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".