The space between the lines of any given text is called the line-height.
This spacing has a big impact on the way the text is read. If the space is too tight, the lines are harder to distinguish the seperate lines. If the space is is too loose the lines will become less coherent. A line-height that's either too tight or too loose will result in an uncomfortable reading experience.
The perfect line-height is different with each font, but should always be between 120% and 145% of the fontsize.
Read more about Line Spacing