This question has been asked a few times and I do agree that the answers are vague so I’ll try to elaborate a bit.
Polling is pinging a website with an HTTP request to receive data back by using an Http:GetAsync request. You would have your discord bot send information to a website that then stores that information, holds it for a set amount of time, and then discards that information after the time established has elapsed - your GetAsync request being sent to this website.
There are a number of ways to go about doing this, however in terms of free methods, I find using Trello for http polling (albeit not the intention of Trello) works well enough for me. I have a list set up that holds cards with data from my discord bot, which then waits 10 seconds for all of my active servers to be able to read it, then deleting the card.