Nitpicks are are unimportant comments, where the code could be merged without even addressing these. These could be things like variable declarations being in alphabetical order, unit tests following a certain structure, or brackets being on the same line.
Good code reviews make it clear when changes are unimportant nitpicks. They usually mark comments like these distinctively, adding the “nit:” prefix to them. Too many of these can become frustrating and take the attention away from the more important parts of the review, so reviewers aim to not go overboard with these.
Better code reviews realize that too many nitpicks are a sign of lack of tooling or a lack of standards. Reviewers who come across these frequently will look at solving this problem outside the code review process. For example, many of the common nitpick comments can be solved via automated linting. Those that cannot can usually be resolved by the team agreeing to certain standards and following them—perhaps even automating them, eventually.