exciting thread title, I know

Can someone please explain secular equilibrium to me? My notes definition says that it means that all the nuclides in a decay series have the same activity. Wikipedia says that it's when the quantity of a radioactive isotope remains constant because its production rate is equal to its decay rate.

For some reason I just can't wrap my head around this. Apparently secular equilibrium can only occur if the half life of the daughter is much shorter than the half life of the parent. But if that's the case, how do you reach equilibrium? Wouldn't the daughter just decay before more daughter material is made? What am I not getting here?

I realized I didn't understand this at all when I couldn't answer the question: "at secular equilibrium, is the number of atoms present larger for the longer-lived isotopes or the shorter-lived isotopes?"

Any help appreciated, I have an exam tomorrow at 10 am :P