Sure, so this changes the gap between elements in the first column to 0rem. The logic behind it is that it looks for all the column elements in the page ([data-testid=column]), then picks the first one (:nth-of-type() selector), and then it selects the [data-testid=stVerticalBlock] element, which is the one that has the gap property. Notice that gap: 0rem does not apply to the checkboxes but to everything placed in that first column, so it should apply for selectboxes or whatever you put in there.
Here is a better explanation on how to find those tags and target particular css properties on an page: