The Hand in the Yu-Gi-Oh! TCG, OCG, and FCG is unplayed cards in your possession that you have drawn from your Deck or otherwise. As implied, you would generally be holding these cards in your hand. You are generally, but not limited to the following, able to Normal Summon or Set the Monsters in your Hand, set the Trap cards in your hand, and Activate or Set the Spell cards in your Hand.
During your Draw Phase after you draw a card it is added to your hand. The hand size limit is six, so during your End Phase if you have more than six cards in your hand, you must discard cards until you have six cards left. extends the hand limit to seven cards, while removes this limit for both players.
Certain cards can interact with cards in either yours, your opponents, or both sets of hands.can discard 1 card from your hand as part of it's effect, and allows you to swap one card from your hand with one card in your opponent's hand.