Condition is the single most important factor in determining a card's value — it describes the physical state of the card, from pristine to heavily damaged. Two copies of the exact same card can be worth wildly different amounts based solely on condition.
The hobby uses a standardized scale to describe condition: Gem Mint, Mint, Near Mint-Mint, Near Mint, Excellent-Mint, Excellent, Very Good-Excellent, Very Good, Good, Fair, and Poor. Each level reflects specific criteria around corners, edges, surface quality, and centering.
Whether you're buying, selling, or submitting for grading, understanding condition is foundational. A trained eye for condition will save you money on purchases, maximize your returns on sales, and help you decide which cards are worth the grading fee.