I’ll go first.
WoW Classic uses layers to help spread out large population of players and prevent any one area of the world from becoming overloaded, which would degrade the play experience. Each layer is a full copy of the entire game world, to provide a sense of continuity with the players around you. When you’re on the same layer as another player, you might see that person repeatedly, in different zones and at different levels, because you’re both on the same layer. The way layering works for groups is fairly straightforward: if you invite someone to group with you and you’re on a different layer than they are, they will be moved from their layer to yours. Thus, when grouping up with someone else on a Hardcore realm, always expect that you’re about to change over to their layer, and be very careful about where you position yourself.
Don’t ever join a group while standing in a cleared murloc camp, or you might find yourself getting viciously mrrrgled.