What is polling?
In one sentence: Asking again and again on a timer whether anything has changed, like checking the letterbox every hour.
A little more
Polling means a program asks the same question on a timer: has anything changed? A news reader polls a feed every hour, an email app might check every few minutes, and a script might poll a web address until a job is finished.
Polling is simple and private, because the website doesn't need to know who is asking, but it has a built-in trade-off: poll often and most checks come back empty; poll rarely and news waits. Cheap replies such as 304 Not Modified keep frequent polling affordable. Long polling is a variation where the server holds the question open until there's an answer.
It's a bit like… Walking to the letterbox every hour to see if a parcel came, even on days when nothing is coming.