why override is required in c#??

my question is quite stupid , but plz answer it.

when we inherit a base class then auto matically we can override all its function . all functions in the derived class , having same name as in base r taken as with keyword new

so i m basically confused then what is the differnce between new n override n y we need override at all bcoz we can override all base class function without keyword override

plz if u know some pdf r doc r link abt this plz provide that

i m jst  a beginner