I take it you mean the "BT Home Hub" - this is a bit of an odd name as a "Hub" in network terms is not the same thing as what th "BT Home Hub" does.
"BT Home Hub" is basicaly a router - In a normaly home setup its job is to give out IP addresses (used to identifiy computers on your network - each one is different) allow all PCs to connect to the internet (where present) and to manage traffic going from one PC to another on the network.
A Switch or Hub is normaly a way of turning 1 network socket into multiple ones (normaly 4, 8, 16, 24 or 48 ports on them) Think of it like a 4 into 1 power extention thing for networks.
Your BT Home Hub probably has 4 network points on it - if you wanted to increase this to say 6 computers on your network (for some epic LAN party action maybe) you could use any 1 of the out puts on the home hub and plug it into the input port on the netgear switch (some switched dont have a specific port to be used as input but most do) you can then use the remaining 3 ports on the home hub for computes plus however many ports are left on the switch.
Remember any PCs conected to the switch - will have to send internet or netwrok trafic to the switch 1st and then on to the home hub - this means internet actions will have a VERY small increased delay - some online gamers can get funny about any kind of extra delay on their conection.