List hygiene basically means regularly removing contacts who are no longer engaging or who are actively hurting your sender reputation — hard bounces, repeated soft bounces, spam complaints, and (for cold outreach especially) people who've never opened or replied after many attempts. ISPs track engagement at the domain and sender level, so a list full of dead or disengaged addresses drags down your reputation even if the emails themselves are fine. A simple habit that helps a lot: after every campaign, remove hard bounces immediately, and periodically prune contacts with zero opens after 5+ sends.