Ironing is an experimental feature in Cura where the top layers are made smoother thanks to the slight flow of the nozzle gliding across the top layer of the print an extra time.

This is an Ironing Test that I created so as to see the difference in the quality of ironing with the different flow percentages

I used the percentages of 10 - 18 %.

To set ironing on in cura, see this youtube video:

https://www.youtube.com/watch?v=3ma0rrY8a08

In cura, you can set the ironing percentages according to height with the support blockers.