Typically they mean whatever someone wanted them to mean when they started the poll. Right?
Most recently the news and politicians discussed that 90% of Americans wanted universal registrations. So that is what the media slams across the stations and papers. But really the poll this comes from asked whether people wanted required background checks for all gun buyers with nothing about mental health additions or the other things stuffed into the proposed bill(s). Really it was 91% and of course they also fail to mention the scope being about 1700 people across a handful of states. According to my trusty microsoft windows calculator, that really means 91% of .00055% of Americans supported background checks for all gun buyers. No matter how enthusiastic you are about those stats… they in no way mean 90% of all Americans.