Perhaps the person means justification. That is where the left margin is even and the right margin is "ragged," or uneven looking, in left alignment. Justified alignment makes both margins straight.
It is a button in Sigil, and you can use format justify when text is selected.
It is also in css, if you want to do it that way. Check out
www.w3schools.com for help on this. I apply justified text to my css for paragraphs. That way all paragraphs are justified.