The Cubs lead the N.L. Central by 1½ games. If the season ended today, they would be the division winner with the third-best record and would play the Atlanta Braves in a division series. The Braves would get home field in that series.
Of course, we hope the Cubs win the division — and at this writing they’re just five games behind the Braves (four in the loss column), so they might even catch them and could have home field in a division series. Note that if the Cubs and Braves do play under this circumstance, no matter who the team with the better record is, they would be NLDS “B” as noted below.