User blog comment:Nayuta Ito/Brainfuck/@comment-25601061-20160716141634/@comment-1605058-20160716165321

It is literally just "take input and put it in current cell". It doesn't really matter at this point how the input it provided - it can be read of some file or the program might wait for the input from the user. In either case, this number is then stored in the cell over which the pointer currently is.