Network protocols often need to transport large chunks of data which are complete in themselves, e.g. when transferring a file. The underlying protocol might not be able to handle that chunk size (e.g. limitation of the network packet size), or is stream-based like TCP, which doesn’t know data chunks at all.
In that case the network protocol has to handle the chunk boundaries itself and (if required) spread the data over multiple packets. It obviously also needs a mechanism to determine the chunk boundaries on the receiving side.
Wireshark calls this mechanism reassembly, although a specific protocol specification might use a different term for this (e.g. desegmentation, defragmentation, etc).
For some of the network protocols Wireshark knows of, a mechanism is implemented to find, decode and display these chunks of data. Wireshark will try to find the corresponding packets of this chunk, and will show the combined data as additional pages in the “Packet Bytes” pane (for information about this pane. See Section 3.20, “The “Packet Bytes” pane”).
Reassembly might take place at several protocol layers, so it’s possible that multiple tabs in the “Packet Bytes” pane appear.
You will find the reassembled data in the last packet of the chunk.
For example, in a HTTP GET response, the requested data (e.g. an HTML page) is returned. Wireshark will show the hex dump of the data in a new tab “Uncompressed entity body” in the “Packet Bytes” pane.
Reassembly is enabled in the preferences by default but can be disabled in the preferences for the protocol in question. Enabling or disabling reassembly settings for a protocol typically requires two things:
The tooltip of the higher level protocol setting will notify you if and which lower level protocol setting also has to be considered.