Enabling forwarding on linux allows network packets from one interface (eth0) to another interface (wlan0). The use for this would most commonly be for setting up your box as a hotspot or bridge. sudo echo 1 > /proc/sys/net/ipv4/ip_forward
If you want to store this permanently edit the file /etc/sysctl.conf
Add the line net.ipv4.ip_forward = 1 to the end of the file.