An ipfs gateway allows people using todays http and https web to easily use the web of tomorrow.
The ipfs team runs a public gateway at ipfs.io, and this can be used for easily playing with ipfs, sharing content on the network to those who don't have ipfs installed locally. You are free to use these of course, but if you wish to run your own gateway, its quite easy to do.
First, you'll want to make sure the machine that you intend to run the gateway on is accessible on the network. If you intend this to be a public gateway, make sure it has a public ip address, if this gateway will just be for a given LAN, make sure its accessible from other computers on that network.
Next, modify your ipfs configuration. The field that needs to be changed is "Addresses.Gateway". The default value is "/ip4/127.0.0.1/tcp/8080". This means that it will only accept connections that come from the same computer that the node is on, on port 8080. You can change the ip address portion to be "0.0.0.0" to allow any other computer to access the gateway. The port can be changed to something else, but it is generally recommended to use a reverse proxy such as nginx to proxy connections from port 80 or 443 to 8080.
With those changes to the config made, simply start the ipfs daemon and try it out!