Hii...
in employment Employer deduct your tax only as TDS on Salary income , they did not file ITR on behalf of you, its your duty to file ITR and disclose all your income along with salary if any.
if you worked under different employers in the same year (between 01 April to 31 March) and not sharing income details from earlier employer with current employer then current employer deduct TDS from your salary assuming you have no earn any income in this year.
for example
if you worked from April to September under Mr. A and got Salary of Rs. 300000 (Yearly 600000)
if you worked from October to March under Mr. B and got Salary of Rs. 400000 (Yearly 800000)
in above case
Mr. A deduct TDS assuming your Estimated income is Rs. 600000(Yearly) and Deduct TDS Monthly upto September
Mr. B deduct TDS assuming your Estimated income is Rs. 400000(Yearly) and Deduct TDS Monthly from October to March)
but your total income in above case is Rs. 700000 and Tax on this amount is much higher due to change of Tax rates slabs
thus you have to pay tax on your total Taxable income earn during the year from all sources