Whenever you visit a website through a link, your device connects to the servers of that website. So if you open a YouTube link, then YouTube sees your device connected to it. This is why people use VPNs because it masks the IP address of the device and makes it more difficult to identify who you are across different websites/apps.

Most websites add trackers to the end of their URLs when you try to share a link. The tracker usually starts after a ? or & symbol. Everything after the ? or & symbol is information that is intended to track and/or identify you. Removing it won’t break the link, but it will prevent the website from connecting your identity to the person sharing it with you.

If people on nostr open your links with trackers, then the website knows there is some kind of connection between you two. The tracker bot on nostr is just telling people to remove the tracker and it provides the link without the tracker for others to open. It’s a very helpful bot for promoting privacy.