Essentially this means that the Income Tax Department found some mismatch between your tax calculations and theirs. You must have received an intimation under Section 143(1) from the Department, showing the mismatch. You can identify the mismatch by comparing it from your Form 26AS.
You must identify the error and submit a rectification request on the Income Tax Department website. A rectification order under Section 154 will be issued after submission and processing.
To know more about how to file a rectification refer to our article here.