A spiral curriculum basically keeps revisiting the same skills over and over with increasing difficulty over a period of time.
IMO, it works well with some subjects, not so much with others.
We use a spiral curriculum approach for history. We did a "surface" study of world history when my kids were younger, then American History, now we are on our second "pass" of world history - this time a little more in depth. We'll cover American history again next year with more depth. Then we'll repeat the process so by the time they finish high school, they will have done a 3rd round of each.
Science is another subject that lends itself to a spiral curriuculum. There are things kids are not ready to learn about in early elementary, but can comprehend in high school. Just because they are young, they don't have to be shut out from the subject entirely.
I'm not convinced it works for math though. For example, my daughter's school used a spiral curriculum for math. They worked on basic addition and subtraction for a few weeks, then they'd take a break from that and work on fractions for a few weeks, then they'd take a break from that and do multiplication (yes, multiplying in 1st grade.) Once they got through several topics, they'd "spiral" back and repeat them....addition and subtraction again, fractions again, ect. She didn't know if she was going or coming, and when I brought her home in 2nd grade, she was struggling with all
the math concepts they had covered.
To me this type of spiral math curriculum is different than "reviewing" math concepts you've already taught and kids have mastered to keep in practice. This particular program "left" addition for several weeks to cover fractions then moved to multiplication. Math skills build on each other, so I'm not sure how you can fully understand multiplication without having a firm grasp on addition.
I discussed this with her teacher who totally agreed with me, but of course had to continue with what the district demanded. Maybe it made the kids "feel smart" if they could multiply at 6 years old?
Hope that helps!