Can someone explain salaried-non exempt to me?

Assuming the company has more than 50 employees and depending on the state:

By being non exempt you do get OT (exempt does not) but if salaried the time and one half OT only comes to you after your 8 hour day (possible 8.5 hours if unpaid for lunch) or because of your rotating schedule, you work more than 160 hours in a 28 day period.

Work periods are normally 28 days because months length changes.

One of my old rotating day off jobs I got 8 hours OT automatically (if I did not take any time off those periods) two times a year

Let me first say that I am not familiar with the term salaried-non exempt unless it means there are no exemptions to the contract which will be drawn up.

You will be paid by the calendar month irrespective of the number of (working) days in that month. Salaried employees do not usually get overtime so the weekends would need to be shared by everyone in your grade. In any case you will be entitled to an employment contract which should contain all these details. If there are any terms in the contract you are unhappy with then you will have to consider if it is worthwhile arguing about them. The job may depend upon your agreeing the terms which other employees of your grade have already agreed

As a non exempt salaried employee you will get paid overtime if you work over 40 hours a week. But a week is defined as a "fixed and regularly recurring period of 168 hours - seven consecutive 24-hour periods." The workweek can start on any day or time as long as the hours are consistently calculated for that same period.

How often you are paid has NOTHING to do with over time exemption or pay rate. It literally ONLY relates to how often you receive pay.

Non-exempt means you are NOT exempt from overtime rules, and should be paid overtime whenever you work more than 40 hours in a pay week. Weekends are NOT overtime by law, except to the extent total hours exceed 40 in a pay week.

You would be entitled to overtime as a non exempt employee. You'll get the same amount per month regardless of 21-23 days, it all evens out.