In a home mesh kit, the extra boxes are nodes. Backhaul is the link that carries the internet connection from the main unit out to those nodes, so the nodes can serve your phones and laptops.
The main unit and the satellites
NETGEAR’s support article on backhaul describes it as communication between the mesh router and its satellites. The internet connection from your modem is plugged into the main unit. The satellites do not each have their own modem. They get connectivity through the backhaul, then offer Wi-Fi to devices nearby.
People also call the satellites nodes. A two-piece or three-piece kit is a main unit plus one or two more nodes. Adding a node does not create a faster plan from your internet provider. It gives the existing connection another place to come out as Wi-Fi. If the plan itself is the bottleneck, see when mesh helps.
Wireless backhaul: dedicated or shared
When there is no cable between the units, the nodes use Wi-Fi to talk to each other. That Wi-Fi has to share the air with everything else, unless the kit sets a radio aside for the job.
NETGEAR’s backhaul article draws that line in plain terms. Higher-performing mesh systems use a dedicated backhaul Wi-Fi band so the link between router and satellites does not compete with phones and laptops. Budget-oriented systems do backhaul and client traffic on the same Wi-Fi band, which can reduce Wi-Fi performance. “Dedicated” here means reserved for node-to-node traffic. “Shared” means your devices and the mesh link take turns on the same band.
How many radios a kit has is the subject of dual-band versus tri-band. A second or third radio is what makes a dedicated backhaul band possible. This page does not assign a speed loss to an extra wireless hop. The sources used here do not give a single percentage that would be true of every house.
Ethernet backhaul
Some mesh systems can use Ethernet cables between the router and the satellites. NETGEAR calls that Ethernet backhaul. The cable replaces the wireless hop for that link.
TP-Link’s Deco support page describes the same idea for its Deco units: the nodes talk over a cable instead of over Wi-Fi. TP-Link says that cable link is generally faster and more stable than a wireless connection between those nodes. It also says that once Ethernet backhaul is up between units, the Wi-Fi backhaul between those units disconnects. A network can still mix wired and wireless backhaul, so one node can be on a cable while another is not.
That is TP-Link’s description of Deco behavior, not a promise that every brand disconnects wireless backhaul the same way, and not a claim that a cable is “always” faster by some fixed amount. If the cable is already in the wall, using it is the mechanism these pages point to. This site does not publish click-by-click setup for a particular app.
How far apart?
The nodes still need a usable link to the main unit or to a neighbor node. If a wireless node is too far away, or too many walls sit between the units, the backhaul itself gets weak and the node cannot help the room it was meant to cover. No source used for this site gives a universal distance in feet or meters that is safe to treat as a rule. Placement ideas that are actually sourced are on when mesh helps.