`printf("%lld\n",((2LL*nCr(n+k+1, k+2))%MOD - n + MOD)%MOD);`

I came across answer to an editorial solution which required the use of modular arithmetic. My solution could not pass some of the test cases because of the absence of + MOD in the above code. Could someone explain me what normalization does using + MOD bring. Which rule of modular arithmetic is being followed?