Basically the area you are in will turn into shards when a high number of people are with you. Shards are basically the same server but phased as to avoid causing issues like latency or so forth.
If you are on a different shard from a friend and they invite you to their group, you will "shard" to their shard and notice that everyone around you disappears etc.
It's new tech that is killing current wow for many ppl such as myself.
Put simple. The server splits into multiple servers for a small area due to higher populations.
P.S. Don't be sorry. A lot of people are still learning about it.