Here is a fairly succinct summary of O’Neill Cylinders in video form.

Even shorter they are a  space staton that rotates to generate artificial gravity by rotating.

(by the way we know that zero gravity is bad for people and the earth normal gravity is  alright. We have no evidence for anything in between. So is half normal gravity half as bad as zero gravity? 20% as bad? 80%? worse on the heart but not as bad on muscle and bones?)

Using an O’Neill Cylinder for an extended time at various fractions  of the equivalent earth gravity would be a way to test the effects on mammals and then humans.