Yu-Gi-Oh! Nightmare Troubadour/Limited and forbidden cards

Because some cards are unbalancing compared to others or allow a broken combination the card game designers did not anticipate the use of some cards is limited.

Restricted cards
The standard maximum of 3 of the same card in your deck. A restricted card can only have 1-2 copies used. If a card is restricted it is a good sign that the card is particularly useful.

Forbidden cards
Later in the game forbidden cards become available for use. However, your deck is limited to one. Because of their overpowered nature it is highly suggested once you get a god card to use that one.