Leading is the amount of vertical space between two lines of text.

In the Design Center, you can change the amount of leading.

Click on the Text Tools tab located at the top of the Design Center. At the top far right, there is a button for Leading that allows you to click up or down to adjust the spacing.

By default, the leading is set at 2. If you change the leading to 1, the space between the two lines of text will decrease.