a network hub
A hub is the simplest, dumbest device for joining several Ethernet cables together. Imagine a person in a room who, the moment they hear anyone speak, shouts the exact same words at full volume to everyone present, with no idea who the message was for. That is a hub: whatever bits arrive on one port get repeated, electrically, out of every other port.
Because a hub works purely at the physical layer, it does not understand frames, MAC addresses, or who is connected. It just regenerates and broadcasts the raw signal. This means every device on the hub hears every transmission, and if two devices send at the same moment, their signals collide and both must back off and retry. All ports therefore share one collision domain and one slice of bandwidth — a 100 Mbps hub with eight busy machines gives each, on average, a small fraction of 100 Mbps.
Hubs were common in the 1990s because they were cheap, but switches made them obsolete. A hub forces half-duplex operation and CSMA/CD, wastes bandwidth by flooding, and lets every machine snoop every other machine's traffic. Today you will mostly meet a hub in a textbook, as the contrast that explains why a switch is better.
Four PCs on a hub: when PC1 sends to PC2, PCs 3 and 4 also receive the bits and must ignore them. If PC3 starts sending at the same instant, the two signals collide and both PC1 and PC3 detect it, stop, and retry after a random wait.
One collision domain for all four ports — the defining weakness of a hub.
A hub is a layer-1 repeater, not a smart device: it makes one big collision domain, so adding machines makes everyone slower. A switch fixes exactly this.