line-height adds room between the lines.

When a paragraph wraps onto several lines, the room each line takes up is its line height. Too little and the lines feel crammed. A bit more makes long text readable.

Give line-height a plain number with no unit, and it works as times the font size. 1.5 on 16px text gives each line 24px of room.

One rule can mix every text property you have met: font, size, weight, alignment and line height. Keep each declaration on its own line so it stays tidy.

<p>Hedgehogs snuffle about at night, looking for beetles, worms and slugs. One hedgehog can wander two miles in an evening.</p>

p {
  font-family: Georgia, serif;
  font-size: 18px;
  line-height: 1.6;
}

Try 1 and then 2 to feel the difference.