It looks like you've got a ham radio transmission!

Here's a breakdown of the information:

* `05:21`: The time of day in UTC (coordinated universal time) when the transmission was sent.

* `JE3USR/P`: The callsign of the amateur radio operator who transmitted this message. In Japan, amateur radio operators' callsigns are typically a combination of letters and numbers that identify their location or region.

+ `J`: Japan

+ `E`: Eastern (or eastern part of) Japan

+ `3USR`: The specific region or prefecture where the operator is located (possibly Tokyo?)

+ `/P`: Indicates that this callsign is for a portable operation, meaning the operator is using equipment outside their usual location.

* `JP-1558(Umami Kyuryo Prefectural Park, JP-NR)`: The location of the transmitter. This is in the format `country-code-number(location-description)`.

+ `JP`: Japan

+ `1558`: A unique identifier for this specific location

+ `(Umami Kyuryo Prefectural Park, JP-NR)`: The name of the park and a hint at its location (Umami Kyuryo is in Nagano prefecture)

* `18102 FT8 FT8 Sent: +13 Rcvd: -01[JH8PFW]`:

+ `18102`: The frequency on which this transmission was sent, in Hz. In this case, it's a VHF (Very High Frequency) band.

+ `FT8`: The mode of transmission used by the operator (in this case, a digital mode that involves exchanging data at regular intervals).

+ `Sent: +13`: The signal strength reported by the transmitter. A positive value indicates a good signal; `-01[JH8PFW]` is likely another amateur radio station's callsign and location.

+ `Rcvd: -01`: The signal strength received by the station that's decoding this message. In this case, it's weak (-01), suggesting some propagation issues.

Keep in mind that ham radio transmissions often involve multiple stations exchanging information, so there may be other operators involved in this conversation that aren't visible here!