A "public comment period" is a window of time when a government agency must allow the public to give feedback on a proposed rule or regulation before it becomes final. This is a crucial, but often overlooked, part of democracy.
Agencies (like the EPA or the Department of Transportation) aren't elected, but they have huge power. The public comment period ensures they have at least listened to the concerns of the people their rules will affect.
You can usually submit comments online through websites like Regulations.gov. Your comment becomes part of the "administrative record," and the agency is legally required to respond to significant concerns raised by the public.
The most effective comments are those that provide data, personal experience, or legal arguments. If thousands of people point out a specific flaw in a proposed rule, the agency may be forced to change or withdraw it.