If you have received a communication of proposed adjustment u/s 143(1)(a) notice, please read the article here. This is different from a 143(1) notice.
While processing your income tax return, the tax department makes small adjustments or corrections in your tax return. They may then recompute your total income and taxes. Such adjustments result in an intimation under section 143(1).
If there is an arithmetical error in the return or certain TDS has been disallowed to be claimed or something has been claimed which is not as per limits allowed — all these result in a notice or intimation under section 143(1). These intimations are usually computer generated.
There may be rounding off errors which the department may correct because of which a final tax due or refund may arise. Sometimes it could just be an intimation that that the Department has determined your refund adjusting for interest.