allowed expenses and disallowed expense

it can't be explained in a single message

Read section 30 to 43D of the Income tax act 1961 .

you can visit
for allotted expenses - https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.raymondbenn.co.uk/services/businesses/detailed-list-of-tax-allowable-expenses&ved=2ahUKEwjb2NTv0MHfAhUJMo8KHZ2DAQ0QFjAOegQIBxAB&usg=AOvVaw2k7jIsSM5Xx-qwhheYr94T&cshid=1545969523158
for disallotted expenses - https://www.google.com/url?sa=t&source=web&rct=j&url=https://cleartax.in/s/expenses-disallowed-pgbp&ved=2ahUKEwjb2NTv0MHfAhUJMo8KHZ2DAQ0QFjAPegQIBhAB&usg=AOvVaw24CRiY8FsDDZPtvc9xpY-l&cshid=1545969523158