“LEADING” IS THE TYPOGRAPHER’S TERM for line spacing – the space between the lines of type on the printed page. The name comes from the thin strips of lead that printers would insert between the lines of metal type in order to space them out. These lead strips were added to prevent the ascenders from one line bumping into the descenders on the line above.
By default, word processors and page layout programs add 20% leading to text, so that a paragraph set in 10pt type will have an extra 2 points of leading added. This is referred to as “10/12pt”, or “10pt on 12pt”.
Beginners often believe that making type larger is the way to make the text more legible. This paragraph is set in 16pt Garamond set “solid”, with no additional line spacing.