I had a very similar question in a job interview! The only difference is that it was an image from SEVIRI on Meteosat.
The imager on HIMAWARI is called the Advanced Himawari Imager (AHI). The AHI IR1 channel is actually channel 13 with a central wavelength of 10.4 µm, which is in the window region (apparently it's called IR1 in reference to an older satellite). The clear-sky atmosphere is mostly transparent in this region of the electromagnetic spectrum, so we can see the surface or clouds mostly unobstructed by water vapour, ozone, or other gases.
In an infrared image such as this one, high clouds are cold. Forecasters who look at such images for a living like to see clouds as white. I'm a physicist and I prefer to map values of high intensity to white, which would make clouds black, but forecasters are the most important users so they get their way. :)
To display cold areas as white, hot areas must logically be displayed as black. So what we are seeing in this image:
- Central Australia is hot. This is expected, because it is a hot desert with high emissivity.
The image is apparently taken at 12:00Z, which should be between 20:00 and 22:00 legal standard time in mainland Australia, so it's not long after sunset, when the land is still hot. Australia should appear less "black" at an image taken 8 hours later. The image is apparently taken at 12:00 "local" (140°E) time (03:00 UTC). It's February and summer in Australia, and the northern hemisphere looks a lot less hot (it's also earlier in the morning there, compared to sunrise).
- Cloud tops are cold. This is expected, because they are in the upper troposphere. Those white clouds are ice clouds.
- There are grey-ish clouds too with little contrast to the ocean. Those are lower, liquid clouds, they are warmer. The lowest clouds may be difficult to identify in infrared images, which is why (for example) night time fog detection from satellites is difficult.
(I got the job!)