Ironing
When you draw a normal layer molten plastic comes out of the hole in the nozzle and then gets squished between whatever is below and the flat area around the nozzle. When you are ironing the head is at the same level as the last layer. A little molten plastic dribbles out as the head moves around. The little bit of molten plastic that dribbles out should fill in any parts of the top layer and be flattened against the flat part of the nozzle around the hole.
There are parameters in the slicer that control how much filament dribbles out, the pattern, and the ever popular more.
Too much plastid and you can get ridges on the top. Too little and it may not fill in all the holes. Different patterns look different. Too fast of too slow and look bad. It’s highly dependant on your printer and how you think it should look.
It will never be the same as the surface against the bed. Think about how the print is orientated to take advantage of the possible flat surface on the bottom of the print.
https://wiki.bambulab.com/en/software/bambu-studio/parameter/ironing