a server is like a TV station
if you want to watch a TV show, like "ER" , you turn your television to channel "12" (a certain TV station)
so if you are on your computer and want certain content , ie a certain web page/server, you tune your "TV" ie your computer to the right channel to get the server/TV station that has the content you want
so for TV
you tune your TV to a certain TV station to watch a certain program
for computers/internet
you "tune" your computer to a certain "server" to get the content you want, ie eBay, yahoo, anandtech, etc