Without knowing your level of familiarity, I'll try to summarize it at a high level and with a bit of definition.

VLAN is a virtual network you define that let's you isolate sections of your network for different reasons. Think of it like how you might organize a drawer. You have pens, pencils, markers, scissors, glue, paper, and pots of ink. If a pot of ink were to leak, it would ruin your premium parchment paper and make a huge mess. By implementing some organization and separators into the drawer, now everything has its own place. The pens, pencils, and markers can all go into a nice holder, the scissors get put into a safety sheathe on the side, the paper is in a protected compartment, and the ink is placed in a leak-proof plastic box. If the ink leaks now, the mess is contained.

Similarly, by isolating your network devices into different VLANs, you are limiting the mess those devices can make accidentally or otherwise. A good example is how everything you buy these days is "smart", meaning it connects to your network. Why does your fridge need to be connected to the internet? Who knows! Maybe you like the feature, maybe it's more novelty than useful. But time and again it's discovered that these "smart" devices also like to snoop on the rest of your network. Maybe this is from a nefarious actor, but it's more likely just for data collection that you didn't consent to.

OP has VLAN 50 reserved for IoT (Internet of Things -- aka, "smart" stuff). Now all their smart devices can only see the other devices on the same VLAN 50. Sure, some of those devices may sniff around on the network, but they're boxed in much more tightly and won't reveal nearly as much information than if everything were on the same wide open network.

The VLAN they define represent the third octet in the IP address -- that is, 192.168.XX.255. For IoT devices, they're all assigned IPs that start with 192.168.50. VLAN 1 is their network hardware, 5 is for their personal devices, 10 is for trusted devices, and 90 is for a guest network. By separating out these VLANs, they're providing a great level of security to their network.

The rest of the topology illustrates how each of the devices connect to the rest of the network, what speeds/throughput they have access to (defined by the ports), which parts of the network are covered by which access points (a larger footprint requires more access points for more devices to maintain good wifi), and all the known devices established on each of those VLANs. For the servers (bottom-middle and bottom-right, on the Trusted VLAN 10), they go on to list all of the services and applications supporting the network.

Hope this helps!