A amateur radio transmission!

Here's a breakdown of the message:

* "23:55" is the time of the transmission in 24-hour format.

* "JF1NDT/3" is the call sign of the transmitting station, indicating:

+ JF: The country code for Japan (in this case, it's actually Japan).

+ 1: The prefix for a specific region or prefecture.

+ NDT: The suffix, identifying the operator.

+ /3: The antenna setup identifier. In this case, "/3" suggests that the station is using an antenna with a gain of 3 dB (a common specification in amateur radio).

* "JP-1575(Hikigawa Prefectural Nature Park, JP-WK)" refers to the location of the transmitting station:

+ JP: The country code for Japan.

+ -1575: A unique identifier for the location.

+ (Hikigawa Prefectural Nature Park): The specific park where the transmitter is located.

+ , JP-WK: The state or region and type of park (in this case, "WK" stands for "Wildlife/Kyotei", a Japanese term).

* "10138 FT8 TNX P2P![JG7MRR]" is the message:

+ 10138: The mode and frequency of transmission in hexadecimal format.

+ FT8: A digital mode specification (more on this later).

+ TNX: An abbreviation for "Thank you", expressing gratitude to the receiving station or a fellow amateur radio operator.

+ P2P: An abbreviation for "Peer-to-Peer", indicating that the transmission was directly between two stations without a repeater or other intermediate node.

+ [JG7MRR]: The call sign of the receiving station, which is not being transmitted back to the original sender. The square brackets around the call sign suggest that it's a separate entity from the transmitting station.

FT8 is a popular digital mode for amateur radio operators, allowing them to exchange information and engage in communication over long distances using minimal equipment.