dp[i][j]=number of ways of choosing j elements out of i elements. First consider any ordering of the elements. Now consider the ith element in this ordering, dp[i][j] can be written as sum of two parts

1. That contains the ith element, so now you have taken 1 element and you need to take remaining j-1 elements from remaining i-1 elements ie dp[i-1][j-1].

2. That do not contain ith element so you need to choose all j elements from the remaining i-1 elements ie dp[i-1][j]