Before you continue to read (or if you want something for reference) please refer to season 1 episode 9.
For a quick recap, in this episode, Ted and Robin go volunteer at a soup kitchen on Thanksgiving where they run into Barney who is there for community service hours. Ted and Robin are tasked with helping sort the good donations from the bad donations which are when they discover that the good donations go to the volunteers and not the poor it is meant to feed.
This leads Ted and Robin to cause a scene which gets them, and Barney who vouched for them kicked out of the soup kitchen. They all wind up at a strip club that Barney says that he usually spends Thanksgiving and Ted meets a stripper which is where the Easter Egg comes in.
Ted begins to talk to a stripper who reveals her real name to Ted which is Tracy. Now, the reason that this is relevant is because the episode ends with a flash forward in time to when Ted is talking to his kids and he is wrapping up the story of that Thanksgiving and he says "and that kids, is how I met your mother".
This is relevant because of the kid's reaction. They were so surprised until Ted told them that it really isn't how he met their mother. Why would the kids be so surprised at that statement if their mother's name, which would not be revealed for many more seasons, wasn't Tracy?
HIMYM dropped this huge hint so early on in the series that I, and I am sure so many other people, brushed off. This was such a huge discovery for me and I was a bit shocked. Go back and rewatch that episode, or series really, and pay attention to that. It all happens towards the end but it is worth the watch.