How do these cards work? {Negate} {Essence Scatter}
To counter a spell is to make it so a spell on the stack does not resolve. If I cast a {Sheoldred, Whispering One} and you cast {Counterspell}, my Sheoldred never hits the field, and goes straight to the graveyard. You can only counter a spell as its being cast; once it hits the field, its no longer a valid target.
Here's the breakdown of counter spells, from the Comprehensive Rules:
701.5. Counter
701.5a To counter a spell or ability means to cancel it, removing it from the stack. It doesn't resolve and none of its effects occur. A countered spell is put into its owner's graveyard.
701.5b The player who cast a countered spell or activated a countered ability doesn't get a "refund" of any costs that were paid.
RESOLVED