When someone says
I stand corrected.
What does that mean?
This question appears to be off-topic. The users who voted to close gave this specific reason:
It means "I admit that I was wrong".
See e.g. Cambridge Dictionaries:
used to admit that something you have said or done was wrong
I stand corrected - the date of foundation was 1411, and not 1412 as I had written.
Said to acknowledge someone who corrects something that one says or writes that was not correct.
The Battle of Hastings was fought in 1056.
No it wasn't! Look in the textbook!
Oh, sorry. I stand corrected - it was fought in 1066.
Note that the Cambridge Dictionary marks this idiom as "formal", and Merriam-Webster's Learner's Dictionary as "somewhat formal".
It means, "I admit that I was wrong".
It is often used by a person who has been holding a strong opinion when faced with conclusive contrary evidence.
John: "My keys are missing. They were stolen!"
Mary: "Look in your pocket."
John (after finding his keys in his own pocket): "I stand corrected."
It's the softest way to say you were wrong before.