The web map fills itself in. It used to show only the ground somebody had already walked, and everything else was black - which looks like a broken map rather than an unvisited one.
Nothing is added to your game. The mod that does it runs on the server and is not in your download.
Your client will connect exactly as it did.
The full list of changes
01Ground made before anybody walks it
The map draws what the server has made and nothing else, so on a young world almost all of it was black - and black is what a map looks like when it is broken, not when it is unexplored. The two were indistinguishable, which is the one thing a map of who owns what must never be.
So the ground is made ahead of time instead of when somebody arrives. It changes nothing about the world itself: the same chunks, generated the same way, at a different hour.
Only the charted half is made. Past the line the map keeps no picture at all, so making ground there would be work thrown away - and it is made anyway the moment somebody walks in.