The Sun has reached the most northerly extent of the annual cycle of the seasons and will now begin the journey to the corresponding southernmost point.
In the north the daylight will begin to shorten and while it is called the start of summer, quite a few of us have been enjoying warmer temperatures for a while compared to earlier in the year. In the south of course the opposite is true.
The time of the June Solstice is (or was) 10:07 UTC, today 21 June 2018.
Thanks to TimeandDate.com for the image.