Decanting allow oxygen to blend into the wine to "open it up" and reach it's full potential. It allows any trapped CO2 in the wine to leave and disperse it's odors along with it. You can decant a wine in various ways. Simply pour a glass and let it sit for a half hour or so will help.
To properly decant though you use a decanter that will sort of splash the wine as it is poured in it to oxygenate the wine. Let it sit for a half hour or so and you will notice a vast difference in most wines. The fruit and other flavors should be much more forward now.The colder the wine is, the longer it should decant but various wines are meant to be enjoyed and certain temps.
I am no expert on this and rarely decant but there are many here that do and I am sure they will chime in and give furthur info.