Why ban or restrict cards?
First, you need to understand that a game stays interesting when there's diversity for players. A deck that's too strong and wins every game quickly becomes boring and repetitive for everyone. Diversity is needed in the cards players choose to build their decks, in the decks they play, and in the formats they can play. The more options there are, the more players there are, and the more interesting the game becomes. It's with this goal of diversity in mind that Wizards of the Coast keeps its Banned and Restricted lists constantly up to date.
It's important to mention that these lists only apply to Constructed formats, not Limited, and that each format has its own specific list.
Banned cards
Banning a card in a format means a player cannot include that card in their deck or their sideboard. When a deck or sideboard contains a banned card, it becomes illegal for official Magic: The Gathering tournaments.
A card gets banned:
1. If, in tournament play, a player must be playing it or has to build their whole strategy around it.
2. When it's too powerful upon entering the battlefield.
3. If its illustration, text, or name is judged to be racist or culturally offensive. As of now, cards deemed racist or culturally offensive are those that invoke an offensive prejudice or stereotype, reference a genocide, include a sensitive religious term, or otherwise contain content deemed culturally insensitive.
Restricted cards
In a tournament, a restricted card is limited to a single copy per player, combining maindeck and sideboard. As of right now, the only format who has a restricted list is Vintage.
Sources:
BANNED AND RESTRICTED LISTS
Depictions of Racism in Magic
