On Christmas morning of 2021, the James Webb Space Telescope successfully launched from Earth. Thomas Zurbuchen, now NASA's associate administrator for science, had made the call. If Webb was going to fail, he would take the blame.

Not only did Webb launch, but its Ariane 5 rocket performed the flight with such precision that the spacecraft was able to save precious fuel for maneuvering, thereby extending its lifetime. Over the next two weeks, engineers and scientists executed hundreds of steps to unfold and fully extend the telescope and its massive sunshield. And then, finally, on Monday, the spacecraft performed one final major burn of its thrusters, falling into a halo orbit around the L2 point.

This means that the Webb space telescope has reached its final destination, a 180-day orbit around this L2 point, which keeps the telescope in line with the Earth as both the instrument and planet orbit around the Sun. Here, while using a minimum amount of fuel to hold its position, Webb can use its sunshield to keep the infrared telescope and its instruments cold.

The work is not done. The telescope has 18 primary mirror segments, which are moved by 132 actuators. These actuators have already been tested and shown to work. Now, over the next three months, telescope operators will fine-tune the alignment of these mirrors. During this process, scientists will use a Sun-like star named HD84406 to focus the mirrors. This star is located about 240 light years from Earth and can be found in Ursa Major near the bowl of the Big Dipper.

At the same time, in the wake of the sunshield, these mirrors and their scientific instruments will continue to cool in order to be able to detect the weak, ultra-distant signals of heat from the Universe's oldest galaxies. //

What is it about HD84406 that makes it the one to use for focusing the mirrors?

There are probably lots of criteria, but I only know two of them:

1. It's in the same 1/3 of space that JWST can see (i.e. the telescope doesn't have to look towards the Sun to see it)
2. It has to be relatively bright (HD84406 is not visible to the naked eye but can be seen with binoculars).

I can't find a reference now, but IIRC it was also selected because it's isolated with nothing behind it that's close (in interstellar terms), so it's easier to determine if the focus is good because there's less background light.