If you say Y here, then your Linux box will be able to act as an
Ethernet bridge, which means that the different Ethernet segments it
is connected to will appear as one Ethernet to the participants.
Several such bridges can work together to create even larger
networks of Ethernets using the IEEE 802.1 spanning tree algorithm.
As this is a standard, Linux bridges will cooperate properly with
other third party bridge products.

In order to use the Ethernet bridge, you'll need the bridge
configuration tools; see Documentation/networking/bridge.txt
for location. Please read the Bridge mini-HOWTO for more
information.

If you enable iptables support along with the bridge support then you
turn your bridge into a bridging IP firewall.
iptables will then see the IP packets being bridged, so you need to
take this into account when setting up your firewall rules.
Enabling arptables support when bridging will let arptables see
bridged ARP traffic in the arptables FORWARD chain.

To compile this code as a module, choose M here: the module
will be called bridge.

If unsure, say N.

source code:

net/bridge/