Card Rulings are official clarifications issued by Wizards of the Coast on the usage of a card.
Usually they give detailed description on specific card characteristics and explain how certain mechanics work including interaction with other cards. Sometimes in-depth articles on wizards.com are linked for further reading.
Card Rulings on the Wiki
If available Card Rulings can be found on each card page beneath the card attributes (mana cost, oracle text, set, etc). Card Rulings are being updated with each new set in Magic: The Gathering Arena. Please note, that series like Commander, From the Vault, etc might bring rulings changes which won't be updated instantly if not included in Magic: The Gathering Arena. You can determine the latest rulings update by the given timestamp. Some of the rulings might refer to the physical card game and will not apply to online play.
If you should encounter errors, ambiguous explanations or simply have questions about a specific card, feel free to leave a comment on the refering card page.