What is happening during the maintenance hour of E-Mini S&P 500?

I’m trying to better understand the trading hours of the E-mini 500 S&P, which according to the CME Groups web pages are shown like this:

This is, however, rather incomprehensible for anyone non-expert.

There (and elsewhere) they say there is a "Daily Maintenance" hour between 4pm - 5pm (Mon-Thu).

Q: What does that mean, and what going on at that time (what significance does it have)?