What does "Isolation" mean in ACID?

Isolation means concurrently running transactions shouldn't affect each other and shouldn't see each other's intermediate results.

Each transaction runs as if it were the only one, even if dozens of operations are happening in the system at once.

Example: if one user is changing an account's balance while another queries it at the same moment, the second one sees the old value until the first transaction fully completes. This prevents errors and data discrepancies.