Banishing cards from the graveyard
There are a lot of zones on the yugioh board. If you look at this playmat (not currently up to date with the newest rules for link summons, but it's fine for this purpose), you can see that there is a "Banished Zone". 

Now, this is not an official zone in Yugioh, since "banished" used to mean "removed from the game" and cards that are banished are supposed to be in "no particular zone". I will however refer to the place where you put banished cards as the "Banished Zone" for simplicity's sake.

To "banish" a card from anywhere means to put the card from wherever it was before faceup into the "Banished Zone". Both players may look at their own or the opponent's face-up banished cards at any time. Each player has his own Banished Zone.

A card in the Banished Zone can usually not be accessed again, but there are some cards that allow you to get back your banished cards.

Cards that interact with banished cards (examples):


Kycoo, the Ghost Destroyer
Necroface
D.D. Warrior Lady
Breakthrough Skill


Card can be banished from anywhere, be it your field, your hand, your graveyard (your discard pile), or your deck. Cards may be banished facedown if a card tells you to do so. You may not look at your opponent's facedown banished cards, but you may look at your own. 

For any further information, see the wiki article on this topic: Banish | Yu-Gi-Oh! Wikia