Condition on a card is four separate judgements that get combined into one word or one number.
The four factors
Centring — how evenly the image sits in its border, front and back separately.
Corners — sharp, or soft, or rounded.
Edges — clean, or chipped. Dark borders show chipping most.
Surface — scratches, print lines, creases, and gloss.
A card can be excellent on three and poor on one, and that one sets the grade.
Graded
Professional grading seals the card in a holder with a number. The common scales run 1 to 10, with the top numbers reserved and the gap between them steep in price.
A graded card has had the argument settled. That is what you are paying the fee for.
Raw
Not graded. Described in words, and the words vary by seller — which is exactly why showing the card properly matters more than the adjective.
Common raw language runs from near mint down through lightly played, moderately played, heavily played, damaged. Use it, and then say what you actually see.
Trimmed
Cut down to improve the edges. It is the serious problem in raw cards and it is settled by measurement, not by eye. If you are selling something valuable and raw, expect to be asked.
What to say if you are not a grader
Say that. "Looks near mint to me but I do not grade" is honest and it protects you from a dispute about a number you were never qualified to assign.
Questions people ask
Is a 9 twice as good as an 8?
Not in condition. Often much more than twice in price.
Does the back count?
Yes, and back centring is the commonly ignored half.
What is a print line?
A manufacturing mark in the surface. Not damage, still affects grade.