It’s a way of setting “rules” for different blocks of days. You can increase or decrease the price, offer a discount for booking 3 days, set different mins and max days for different check-in days, even just set multiple days they can check in or out - a bunch of stuff. You use it with your Multi-calendar. That looks like this:
You can make the rules by selecting blocks of dates for all, some or just one of the listings. I can select all of the empty dates and give them the same rule, maybe a discount if I want to book them or a minimum stay if I don’t want to book them (effectively blocking them). Like this:
These are the things you can set, they each open to a bunch of options. You can make all kinds of combinations of these options to create very specific rules (if this, then that, but only here). You’ll see in the last photo that you can “Save the Rule Set” and give it a color to code it to use it later or repeatedly.
On the multicalendar, you can “show rule sets” and they’'ll all come up with their different colors and such.
I used it a ton over the summer when it was super-busy. Sometimes I’d get a block of 4 days stranded together in the month and would prefer to rent all 4 of the days together instead of only 1 or 2 (depending on that listing’s minimum) so I could set a Rule Set that still allowed them to be booked individually (because a booking is a booking!) but the price would decrease if you booked 3 and then decrease a little more if you booked all 4 of them. It worked really well! And I liked it because, if I was going to end up flipping the unit more often, I’d also be making more money, so it seemed more worth it.
Also, in my city, we have a few random Saturdays that are super-high demand because of a recurring event. My Saturdays are 2-day minimums but on these Saturdays I’d prefer you book Friday/Saturday because I get a ton of $ for that but some people would book Saturday/Sunday to save money and then leave my Friday in a position where the price couldn’t stay as high, so for these Saturdays, I set a rule that if you were only booking 2 days you couldn’t check-in on Saturday forcing the Friday to go with the Saturday unless you were booking at least 4 days (then you could check-in on Saturday because now you’ve also booked my Mon and Tue and that helps me out in a different way).
Honestly, it turns into Algebra, which I really enjoy 
but ultimately you can use them to manipulate guests into booking the way you’d prefer without limiting bookings overall (e.g. setting a straight 4-day minimum at all times).
Anyways…sometime in late August something went awry. We stopped getting bookings and it didn’t make any sense (in late January it would have taken me a lot longer to notice ,) and it wasn’t any of the usual suspects (we were showing up in searches, there wasn’t a loss of a new-listing bump, no decease in ratings, location was correct, no new hotels, no new legislation, etc, etc). We were super-busy at the time so I only did some skimming in the Community Center but found some reports from other hosts (several, in different parts of the world) that were reporting a similar situation and someone showed that there was a possible bug in the mult-calendar rule sets, something to do with a “various rule sets” that you could find by searching. They removed their rule sets and everything seemed to be fixed for them. I also found this “various” message in my rule sets so deleted all my past and future rule-sets and got 3 bookings in the next hour and continued normally since. I never followed-up (still meaning to) but it seemed to fix my problem and I probably don’t really need them again until next summer anyways. I’ve also learned to use the few rules available in the regular settings to their max potential.
This is super-way-too-long and I’ve probably lost you but let me know if you have any specific questions, I’m happy to help!