Section 14a r.w rule 8d

It says any expense incurred to earn exempt income u/s 10 for say dividend is not allowed to be claimed as deduction

I will explain basic overview of sec 14A rule 8D..
any amount incurred for getting exempted income cannot be allowed as expenditure.. for example you are doing business having turnover of Rs.500000 and business expenditure Rs.220000 and as well as u have Dividend income of Rs.100000... for getting Dividend u incurred Rs.5,000...
sec 14A rule 8D says that's Rs.5000 is not allowed as business expenditure so your net taxable income is only 2.8 lakhs(5L-2.2L)..